I often see posts requesting "more on naval advice," so I'm throwing my two cents out there.
Obviously, on a pangea map you can completely ignore navies (not saying you should). On the maps where they're required a strong navy can be sooo much fun... and make defense easy.
In my experience (somewhat limited) I haven't seen the AI make a strong run for Astronomy and Chemistry. Once they get the technology, they don't work too hard to produce frigates and galleons to control the seas. So even without a technology and/or production advantage you can still have the strongest navy... and with the strongest navy you don't need the strogest army.
Once at this point I consistenly succeed with a simple strategy: always pair up your ships. Two galleons will almost never lose to a single enemy galleon. Same follows when you get frigates. The AI usually won't attack your pair of ships unless they have a superior force. At the beginning of a war you can quickly eliminate all the lone ships to secure a numerical advantage, then blockade enemy ports or patrol open waters to ensure that enemy attacks never make landfall. It's way cheaper and easier to sink a boat than to destroy the army inside it. If you have to sacrifice ships to achieve this it's still worth it, so long as you have more ships than the enemy.
I've seen the subject of Ironclads come up a few times. Usually it's a discussion about how they're not very useful, and most people skip them. Assuming you're on a map that requires you to cross open ocean, the ironclad is reduced to mostly defense. I usually build a pair of them to demolish any enemy ships that make it to my coast. By that point the enemy has probably landed troops... but if I can destroy the ships that brought them I'll still consider it a small victory.
If you can beat an enemy navy once, they'll have a very hard time building up enough to challenge you again. You'll have experienced ships and they won't. Once again, the AI falters here. They'll have a whole army sitting around unused while they send out a couple ships at a time to be annihilated by a superior navy.
In combat, I usually attack with my less experienced ship first. If you lose, your more experienced ship can finish the job and continue to promote into a flagship that can never be challenged. Keep those highly experienced frigates around until they can be upgraded to destoryers, and the ocean becomes a shooting range.
I haven't dealt with modern naval encounters much. One harsh lesson that I learned: If your opponent gets destroyers first... all the frigates in your fleet can't beat them. Modern navies the AI seems better at managing. They will stack a lot more ships together so you can't use just two ships to do the job.
Obviously, on a pangea map you can completely ignore navies (not saying you should). On the maps where they're required a strong navy can be sooo much fun... and make defense easy.
In my experience (somewhat limited) I haven't seen the AI make a strong run for Astronomy and Chemistry. Once they get the technology, they don't work too hard to produce frigates and galleons to control the seas. So even without a technology and/or production advantage you can still have the strongest navy... and with the strongest navy you don't need the strogest army.
Once at this point I consistenly succeed with a simple strategy: always pair up your ships. Two galleons will almost never lose to a single enemy galleon. Same follows when you get frigates. The AI usually won't attack your pair of ships unless they have a superior force. At the beginning of a war you can quickly eliminate all the lone ships to secure a numerical advantage, then blockade enemy ports or patrol open waters to ensure that enemy attacks never make landfall. It's way cheaper and easier to sink a boat than to destroy the army inside it. If you have to sacrifice ships to achieve this it's still worth it, so long as you have more ships than the enemy.
I've seen the subject of Ironclads come up a few times. Usually it's a discussion about how they're not very useful, and most people skip them. Assuming you're on a map that requires you to cross open ocean, the ironclad is reduced to mostly defense. I usually build a pair of them to demolish any enemy ships that make it to my coast. By that point the enemy has probably landed troops... but if I can destroy the ships that brought them I'll still consider it a small victory.
If you can beat an enemy navy once, they'll have a very hard time building up enough to challenge you again. You'll have experienced ships and they won't. Once again, the AI falters here. They'll have a whole army sitting around unused while they send out a couple ships at a time to be annihilated by a superior navy.
In combat, I usually attack with my less experienced ship first. If you lose, your more experienced ship can finish the job and continue to promote into a flagship that can never be challenged. Keep those highly experienced frigates around until they can be upgraded to destoryers, and the ocean becomes a shooting range.
I haven't dealt with modern naval encounters much. One harsh lesson that I learned: If your opponent gets destroyers first... all the frigates in your fleet can't beat them. Modern navies the AI seems better at managing. They will stack a lot more ships together so you can't use just two ships to do the job.
You could hardly have a more different approach from mine.
) and then let lose the wolf packs. I did that to Monte in a game where he was being his usual objectional self (I kept saying NO to his silly demands early on so he hated me) and sank all his old sailing fleet and pillaged his seafood. Very satisfying 
and his buddy Hanibal couldn't do a thing about it without going to war with me.
