You may want to hold off on permitting any civilization to begin in an ocean. The game will NOT generate sites underwater, even if they appear on maps, in legends, have effects on worldgen, and have a population including important historical figures. If you were to go there, you would find water and hungry fish, and any adventurer attempting to begin a game in such a place ends up as some cavern creature deep underground. The only exception being if the sea is normally frozen, it seems, and then there's a chance that the game will be a douche and unfreeze the water for just long enough for you to fall in, then freeze it again.
I've found, however, that it is entirely possible to have a civ begin in say, a wetlands environment, and prefer oceans/lakes 10:1 over every other environment. They will immediately move to nearby coastlines (if any) and build cities along the shores for miles in every direction, but will only resort to attempting to place sites out on the water if there's no room along the coast. I had a very similar civilization in a mod I had uploaded for 31.25. Not sure If I'll ever bother to update it seeing as how pumping the game full of crap now results in reaching the Windows RAM limit way too fast.
I look forward to the day Toady permits artificial islands and Atlantis/Venice-style cities in bodies of water.