Introduction
I wanted to write this guide because it contains elements of Civ 5 that are rarely seen in higher level play. The Ottomans are widely regarded as having one of the worst unique abilities, and navies in general are seen as underpowered. Current domination builds focus primarily on timing: getting a certain number of a certain unit before a certain time, in order to crush nearby enemies before they get the tech to counter it. And no one seems to pick the Honor policy branch any more. This guide will be quite different in these regards, and will hopefully make a refreshing change of pace for anyone following it.
As you probably expect, this build won't work well on maps with continent-sized landmasses. You could either pick an islandy map to try it out, or if you're like me and like to play with all random settings, pull it out of your repertoir whenever the conditions seem to be suitable. Any civ can do it fine, although Ottomans and England certainly have sizeable advantages. It will work fine on any difficulty, as the AI bonuses - higher tech and numerical advantage - don't counter ships particularly well.
The Key to Naval Power
In my opinion, naval logistics (two shots per turn) is by far the most powerful promotion across all units in the game. Double attack is more powerful for ships since they have more movement points to use it, but more importantly, logistics lets units move after attacking, which not only makes ships immune to land-based counterattack, but also overcomes the one-unit-per-tile limit by allowing multiple units to shoot from the same tile in the same turn.
Here is a comparison to demonstrate my point. An unpromoted caravel is about as good as an unpromoted crossbowman: both do minor damage, the crossbowman fights better, but the caravel has substantial scouting abilities. A caravel with logistics is about as good as a keshik with logistics: both do damage without getting hit, the keshik does more, but the caravel is much harder to kill and still has its scouting abilities. Caravels also get to keep their promotions upon upgrade, while crossbowmen and keshiks don't.
So far, getting logistics improves a caravel's worth from a crossbowman to a keshik, but it gets better. A fleet of 10 caravels with logistics is about as good as an army of 10 artillery with logistics. Before you call me a liar, keep in mind that an army of 10 artillery with logistics is about as good as an army of 5 artillery with logistics, since any more just get in the way (of course, you could split them into 2 armies of 5 artillery, but my point is that naval logistics overcomes one-unit-per-tile limits). Add in shots wasted moving and setting up, and in the long-run average, only 2-3 of your 10 artillery will actually shoot in a turn. This kills about 2 infantry a turn, which is also what 10 caravels will kill.
One step further, and a fleet of destroyers with logistics is so powerful I can't think of a land equivalent. They are unkillable except by missiles or nukes - land units can't reach them, naval units will land one shot and get sniped instantly, and while I haven't tested it against aircraft (mainly because once you get your destroyer fleet, the game is pretty much over), destroyers are designed to be anti-air units, so I assume the aircraft would lose badly. They also have 10 (!) moves and 9 (!!) sight, assuming they're upgraded from caravels and you captured the Great Lighthouse, and 8 of them will bring any city to 1HP in 2 turns.
The Opening
If ever there was a situation to go straight for National College, this is it. Your goal, Astronomy, is a renaissance tech with Writing as a prerequisite, and you won't have much room for expansion anyway. I usually tech Pottery->Writing and build Monument->Granary->Library->National College, as there's not much need for an early Scout on island maps (on random maps, you can usually figure out you're on an island immediately). For social policies, nothing in Tradition or Liberty help much, so get Honor and go straight for the 1.5x XP policy, and then Commerce (make sure you hit medieval in time) for +1 move/+1 sight and +3 hammers in coastal cities. I still haven't figured out what to do with the Great General (ships don't get their bonus), but since we're demonstrating unusual stuff here, you could very well make a citadel to make yourself completely safe against land-based aggression.
Once writing finishes, get your resource techs, Sailing to explore (and capture barbarians) with triremes, and Philosophy for a temple, which is much more important without Tradition/Liberty to increase your culture. From there, just get to Astronomy however you like. If you like RA blocking, you've probably already figured out you can get Education and Astronomy from RAs around turn 100. If you don't like RA blocking, it'll probably happen around turn 150. That seems like a huge difference, but being late isn't too bad, as you'll have more gold to purchase caravels, and also more barbarian caravels to capture. I definitely wouldn't recommend non-Ottoman no-RAs, though.
National College will finish around turn 40-50, after which you can build anything you like. As I mentioned above, a Temple is more important for an Honor-based strategy. You definitely want a worker, although Optics will usually get you one from a barbarian camp on a nearby island with a city-state. Settling more cities is certainly a good choice, but being an island map, sometimes there aren't any good spots. Getting Heroic Epic up is nice, since ships can get Morale (they don't get barracks/armory XP), although it isn't vital for a strategy where you always do 1 damage per shot and never get hit (suggestion by Lokos451). If you really run out of stuff to do, you can build wonders for the gold you get when someone beats you to it. Just try to get a harbour up before Astronomy, as you'll be building a lot of boats.
The First Attack
Make as many caravels as you can, purchase them if possible, and try to capture every barbarian ship you see. If you haven't played with this ability before, it works in a strange way: when you move your ship into a tile adjacent to a barbarian ship, it has a 50% chance of converting. If it doesn't convert immediately, it will never convert, so kill it for experience. You should also bombard coastal barbarians for experience; anything that gets you closer to logistics is helpful. You'll also need a scout (if you're not familiar with the mechanic, ships can't capture cities, they can only bring it down to 1HP, while any melee unit automatically wins against a 1HP city - in this case, you will keep your scout embarked and attack from the water when the city gets down to 1HP).
Once you have 5-10 ships (how many you need depends on your target), it's time to attack! You need to pick your first target city carefully, and make sure your navy is in position before the attack commences. Your target should have plenty of ocean tiles next to it, so that your ships can all attack at once. It should have minimal ranged defenders (try to avoid trebuchets, cannons, caravels or frigates). It should also have a second target city nearby that is also not too heavily defended. The goal is to take it without losing any caravels. However, make sure it can't be retaken by the enemy the next turn, otherwise you'll lose your scout (it's fine to lose it the turn after that, since then you can save your scout).
Any ship you have will do 1 damage to a city. Sometimes a caravel will do 2 damage, but don't count on it. Thus, the Ottoman mass of captured galleys and triremes are just as good as caravels at attacking, although they are much less manoeuvrable. Furthermore, the AI prioritises weaker targets, so they're going to keep attacking your worthless ships (especially if you can use Heal Instantly on them), while your caravels survive to get the XP they need for logistics. If you're not playing Ottomans, you should have built 2 triremes to explore; these should be enough cannon fodder to take your first city, although you might need to wait for more caravels or pick a weaker target.
Once you take your first city, your ships can heal, and life becomes much easier. Your damaged ships should heal up while your undamaged ones farm ground units for XP. Then, proceed to take the second target the same way, only this time you can swap damaged caravels in and out from the healing grounds next to the first target, which means you can accomplish this even without any cannon fodder. If you're playing Ottomans, you're likely to have a large surplus of crappy boats at this point; I like to sacrifice them, since otherwise I'd probably just delete them to save upkeep anyway.
A key ingredient to success is XP farming. You need 100XP to get logistics. Shooting a city gives 4XP (with the Honor 1.5x XP policy), unless the city is already at 1HP in which case it gives no XP, shooting a unit gives 3XP, and killing an embarked unit by moving into it gives 1XP. You should always be on the lookout for maximising XP on your caravels. Maybe you can take the city this turn, but if you wait a turn, they can each get another round off. Maybe you can kill a land unit outright, but spreading out your fire lets them survive and heal, thereby giving more XP. Maybe you can instantly kill an embarked unit, but if you shoot it with 3 caravels (it always does 4 damage), you get 9XP out of it instead of 1. If things went well, your caravel fleet should have logistics, or be very close, after taking this second city and mopping up the defenders. If not, keep fighting until they have it.
The Endgame
Notice that I left out the middle game. Unlike land-based rushes, there is no middle game, as a successful opening generally leads to a situation where you can win as long as you don't make any mistakes, regardless of what the other players do. Many will blame the AI's ineptness at naval combat, but I think it's more a fundamental imbalance in Civ 5's naval system. There simply isn't anything in the game to counter naval superiority, as land units can't reach, and naval units are outnumbered by definition. Barring misclicks, a counterattack is pretty much the only thing that can go wrong, but with a citadel, that's not likely to work either.
Anyway, once you have your logistics fleet up (I find a good number to start is 4 caravels for Ottomans, 6 caravels for non-Ottomans), it's easy going. You should continue to build/capture ships, training them to logistics on random land units (your caravels who already have logistics should move on and save them for new ships to farm). Capture the city with the Great Lighthouse if you can. Slowly tech towards Electricity for destroyers. Don't bother with frigates; they're more expensive, slower, and still only do 1 damage to cities.
Watch out for England. A frigate will absorb a fair bit of damage and can do some damage to you; a ship of the line might kill a damaged caravel instantly. Longbowmen carpets are also nasty. I usually wait until England's unique units become obsolete before I attack them. In fact, picking England yourself is powerful as much for their bonuses as for ensuring no one else gets them. Of course, watch out for artillery too. Just hang back out of range until you picking these dangerous units off, and don't send your non-logistics caravels against them.
Happiness might be a problem. Unlike land-based domination, however, you can just trade away unnecessary cities, as they won't cut off your trade network or your reinforcements. The AI will also sometimes pay huge amounts of money to get one of their former cities back. You don't even need to hold onto capitals; as long as you're the only remaining player who hasn't ever lost control of their capital, you win. In theory, you could just go around sniping capitals and gifting them back immediately.
I wanted to write this guide because it contains elements of Civ 5 that are rarely seen in higher level play. The Ottomans are widely regarded as having one of the worst unique abilities, and navies in general are seen as underpowered. Current domination builds focus primarily on timing: getting a certain number of a certain unit before a certain time, in order to crush nearby enemies before they get the tech to counter it. And no one seems to pick the Honor policy branch any more. This guide will be quite different in these regards, and will hopefully make a refreshing change of pace for anyone following it.
As you probably expect, this build won't work well on maps with continent-sized landmasses. You could either pick an islandy map to try it out, or if you're like me and like to play with all random settings, pull it out of your repertoir whenever the conditions seem to be suitable. Any civ can do it fine, although Ottomans and England certainly have sizeable advantages. It will work fine on any difficulty, as the AI bonuses - higher tech and numerical advantage - don't counter ships particularly well.
The Key to Naval Power
In my opinion, naval logistics (two shots per turn) is by far the most powerful promotion across all units in the game. Double attack is more powerful for ships since they have more movement points to use it, but more importantly, logistics lets units move after attacking, which not only makes ships immune to land-based counterattack, but also overcomes the one-unit-per-tile limit by allowing multiple units to shoot from the same tile in the same turn.
Here is a comparison to demonstrate my point. An unpromoted caravel is about as good as an unpromoted crossbowman: both do minor damage, the crossbowman fights better, but the caravel has substantial scouting abilities. A caravel with logistics is about as good as a keshik with logistics: both do damage without getting hit, the keshik does more, but the caravel is much harder to kill and still has its scouting abilities. Caravels also get to keep their promotions upon upgrade, while crossbowmen and keshiks don't.
So far, getting logistics improves a caravel's worth from a crossbowman to a keshik, but it gets better. A fleet of 10 caravels with logistics is about as good as an army of 10 artillery with logistics. Before you call me a liar, keep in mind that an army of 10 artillery with logistics is about as good as an army of 5 artillery with logistics, since any more just get in the way (of course, you could split them into 2 armies of 5 artillery, but my point is that naval logistics overcomes one-unit-per-tile limits). Add in shots wasted moving and setting up, and in the long-run average, only 2-3 of your 10 artillery will actually shoot in a turn. This kills about 2 infantry a turn, which is also what 10 caravels will kill.
One step further, and a fleet of destroyers with logistics is so powerful I can't think of a land equivalent. They are unkillable except by missiles or nukes - land units can't reach them, naval units will land one shot and get sniped instantly, and while I haven't tested it against aircraft (mainly because once you get your destroyer fleet, the game is pretty much over), destroyers are designed to be anti-air units, so I assume the aircraft would lose badly. They also have 10 (!) moves and 9 (!!) sight, assuming they're upgraded from caravels and you captured the Great Lighthouse, and 8 of them will bring any city to 1HP in 2 turns.
The Opening
If ever there was a situation to go straight for National College, this is it. Your goal, Astronomy, is a renaissance tech with Writing as a prerequisite, and you won't have much room for expansion anyway. I usually tech Pottery->Writing and build Monument->Granary->Library->National College, as there's not much need for an early Scout on island maps (on random maps, you can usually figure out you're on an island immediately). For social policies, nothing in Tradition or Liberty help much, so get Honor and go straight for the 1.5x XP policy, and then Commerce (make sure you hit medieval in time) for +1 move/+1 sight and +3 hammers in coastal cities. I still haven't figured out what to do with the Great General (ships don't get their bonus), but since we're demonstrating unusual stuff here, you could very well make a citadel to make yourself completely safe against land-based aggression.
Once writing finishes, get your resource techs, Sailing to explore (and capture barbarians) with triremes, and Philosophy for a temple, which is much more important without Tradition/Liberty to increase your culture. From there, just get to Astronomy however you like. If you like RA blocking, you've probably already figured out you can get Education and Astronomy from RAs around turn 100. If you don't like RA blocking, it'll probably happen around turn 150. That seems like a huge difference, but being late isn't too bad, as you'll have more gold to purchase caravels, and also more barbarian caravels to capture. I definitely wouldn't recommend non-Ottoman no-RAs, though.
National College will finish around turn 40-50, after which you can build anything you like. As I mentioned above, a Temple is more important for an Honor-based strategy. You definitely want a worker, although Optics will usually get you one from a barbarian camp on a nearby island with a city-state. Settling more cities is certainly a good choice, but being an island map, sometimes there aren't any good spots. Getting Heroic Epic up is nice, since ships can get Morale (they don't get barracks/armory XP), although it isn't vital for a strategy where you always do 1 damage per shot and never get hit (suggestion by Lokos451). If you really run out of stuff to do, you can build wonders for the gold you get when someone beats you to it. Just try to get a harbour up before Astronomy, as you'll be building a lot of boats.
The First Attack
Make as many caravels as you can, purchase them if possible, and try to capture every barbarian ship you see. If you haven't played with this ability before, it works in a strange way: when you move your ship into a tile adjacent to a barbarian ship, it has a 50% chance of converting. If it doesn't convert immediately, it will never convert, so kill it for experience. You should also bombard coastal barbarians for experience; anything that gets you closer to logistics is helpful. You'll also need a scout (if you're not familiar with the mechanic, ships can't capture cities, they can only bring it down to 1HP, while any melee unit automatically wins against a 1HP city - in this case, you will keep your scout embarked and attack from the water when the city gets down to 1HP).
Once you have 5-10 ships (how many you need depends on your target), it's time to attack! You need to pick your first target city carefully, and make sure your navy is in position before the attack commences. Your target should have plenty of ocean tiles next to it, so that your ships can all attack at once. It should have minimal ranged defenders (try to avoid trebuchets, cannons, caravels or frigates). It should also have a second target city nearby that is also not too heavily defended. The goal is to take it without losing any caravels. However, make sure it can't be retaken by the enemy the next turn, otherwise you'll lose your scout (it's fine to lose it the turn after that, since then you can save your scout).
Any ship you have will do 1 damage to a city. Sometimes a caravel will do 2 damage, but don't count on it. Thus, the Ottoman mass of captured galleys and triremes are just as good as caravels at attacking, although they are much less manoeuvrable. Furthermore, the AI prioritises weaker targets, so they're going to keep attacking your worthless ships (especially if you can use Heal Instantly on them), while your caravels survive to get the XP they need for logistics. If you're not playing Ottomans, you should have built 2 triremes to explore; these should be enough cannon fodder to take your first city, although you might need to wait for more caravels or pick a weaker target.
Once you take your first city, your ships can heal, and life becomes much easier. Your damaged ships should heal up while your undamaged ones farm ground units for XP. Then, proceed to take the second target the same way, only this time you can swap damaged caravels in and out from the healing grounds next to the first target, which means you can accomplish this even without any cannon fodder. If you're playing Ottomans, you're likely to have a large surplus of crappy boats at this point; I like to sacrifice them, since otherwise I'd probably just delete them to save upkeep anyway.
A key ingredient to success is XP farming. You need 100XP to get logistics. Shooting a city gives 4XP (with the Honor 1.5x XP policy), unless the city is already at 1HP in which case it gives no XP, shooting a unit gives 3XP, and killing an embarked unit by moving into it gives 1XP. You should always be on the lookout for maximising XP on your caravels. Maybe you can take the city this turn, but if you wait a turn, they can each get another round off. Maybe you can kill a land unit outright, but spreading out your fire lets them survive and heal, thereby giving more XP. Maybe you can instantly kill an embarked unit, but if you shoot it with 3 caravels (it always does 4 damage), you get 9XP out of it instead of 1. If things went well, your caravel fleet should have logistics, or be very close, after taking this second city and mopping up the defenders. If not, keep fighting until they have it.
The Endgame
Notice that I left out the middle game. Unlike land-based rushes, there is no middle game, as a successful opening generally leads to a situation where you can win as long as you don't make any mistakes, regardless of what the other players do. Many will blame the AI's ineptness at naval combat, but I think it's more a fundamental imbalance in Civ 5's naval system. There simply isn't anything in the game to counter naval superiority, as land units can't reach, and naval units are outnumbered by definition. Barring misclicks, a counterattack is pretty much the only thing that can go wrong, but with a citadel, that's not likely to work either.
Anyway, once you have your logistics fleet up (I find a good number to start is 4 caravels for Ottomans, 6 caravels for non-Ottomans), it's easy going. You should continue to build/capture ships, training them to logistics on random land units (your caravels who already have logistics should move on and save them for new ships to farm). Capture the city with the Great Lighthouse if you can. Slowly tech towards Electricity for destroyers. Don't bother with frigates; they're more expensive, slower, and still only do 1 damage to cities.
Watch out for England. A frigate will absorb a fair bit of damage and can do some damage to you; a ship of the line might kill a damaged caravel instantly. Longbowmen carpets are also nasty. I usually wait until England's unique units become obsolete before I attack them. In fact, picking England yourself is powerful as much for their bonuses as for ensuring no one else gets them. Of course, watch out for artillery too. Just hang back out of range until you picking these dangerous units off, and don't send your non-logistics caravels against them.
Happiness might be a problem. Unlike land-based domination, however, you can just trade away unnecessary cities, as they won't cut off your trade network or your reinforcements. The AI will also sometimes pay huge amounts of money to get one of their former cities back. You don't even need to hold onto capitals; as long as you're the only remaining player who hasn't ever lost control of their capital, you win. In theory, you could just go around sniping capitals and gifting them back immediately.
