Okay, so everybody knows that naval combat in FFH is pretty tame. I'm sure many people have ideas of ways to spice it up... for starters, just more naval units, and a bunch of naval UUs would go a long way to encouraging diversity.
But you'd still have the problem that ships take a long freaking time to build and don't really accomplish much other than fighting other ships. So why not make it easier to mass produce them? Say, a national wonder that halves their production time?
Maybe a World Ritual, something similar to Nereid Alliance, which causes Nereids to occasionally spawn in the coastal cities of the builder? Tritones would also work... it'd be cool if both were in the game somewhere, although not using the same mechanic.
It only takes two civilizations going for naval supremacy for the fighting to get pretty intense, so all you'd need to do is add a Good (or Neutral) aligned obvious enemy for OO (Nereids fit, I think, as would Mermen and some sort of dolphin thing), and buff the navies of a couple civs that aren't Lanun, and you're set. That said, here's some straight-out-of-my-ass possible benefits for a few of the civs:
Luchiurp: "Battleship" UU with slow movement but impressive strength.
Doviello: "Longboat" UU, low strength but incredibly low cost, easy to spam, perfect for transporting a large army of raiders but vulnerable to attack
Amurites: "Arcane Vessel," a UU upgradable from Arcane Barge which gets +1 strength and Channeling II.
Khazad: Dwarven Juggernaut, UU using Dwarven Cannon technology, giving it average strength for late-game naval, but massive bombard and extra collateral.
Both elven civs: Elven Sails, UU replacing Dragonboat, +1 movement.
Kuriotates: Ichthyocentaur, basically centaurs that can swim.
Anyone else got any ideas?
Really, what we need most of all is a lot of Barbarian spawning in the water. Maybe add a new early game barbarian naval unit that spawns often and dies easy, fulfilling the double purpose of requiring you to protect your workboats, and feeding much needed XP to your developing navy? I'd go with Undines for this... water spirits that drag you under so you drown; the scourge of swimmers and fishermen, but not much of a threat to a warship.
But you'd still have the problem that ships take a long freaking time to build and don't really accomplish much other than fighting other ships. So why not make it easier to mass produce them? Say, a national wonder that halves their production time?
Maybe a World Ritual, something similar to Nereid Alliance, which causes Nereids to occasionally spawn in the coastal cities of the builder? Tritones would also work... it'd be cool if both were in the game somewhere, although not using the same mechanic.
It only takes two civilizations going for naval supremacy for the fighting to get pretty intense, so all you'd need to do is add a Good (or Neutral) aligned obvious enemy for OO (Nereids fit, I think, as would Mermen and some sort of dolphin thing), and buff the navies of a couple civs that aren't Lanun, and you're set. That said, here's some straight-out-of-my-ass possible benefits for a few of the civs:
Luchiurp: "Battleship" UU with slow movement but impressive strength.
Doviello: "Longboat" UU, low strength but incredibly low cost, easy to spam, perfect for transporting a large army of raiders but vulnerable to attack
Amurites: "Arcane Vessel," a UU upgradable from Arcane Barge which gets +1 strength and Channeling II.
Khazad: Dwarven Juggernaut, UU using Dwarven Cannon technology, giving it average strength for late-game naval, but massive bombard and extra collateral.
Both elven civs: Elven Sails, UU replacing Dragonboat, +1 movement.
Kuriotates: Ichthyocentaur, basically centaurs that can swim.
Anyone else got any ideas?
Really, what we need most of all is a lot of Barbarian spawning in the water. Maybe add a new early game barbarian naval unit that spawns often and dies easy, fulfilling the double purpose of requiring you to protect your workboats, and feeding much needed XP to your developing navy? I'd go with Undines for this... water spirits that drag you under so you drown; the scourge of swimmers and fishermen, but not much of a threat to a warship.
Just the way you proposed it. 
(Just not every single game though...)
