Not quite sure how trivial it is but it turns out the game actually checks to see if a creature is amphibious or not when placing a creature's civ in an ocean biome--I discovered this when I made a generic animal people civ and added axolotl people to the list of possible creatures (along with adder men, hamster men, kea men, olm men, kakapo men, loon men, hare men, and jumping spider men because I like those animals) and set the biomes to these parameters.
[SETTLEMENT_BIOME:ALL_MAIN:1]
[SETTLEMENT_BIOME:NOT_FREEZING:2]
[SETTLEMENT_BIOME:ANY_FOREST:3]
[BIOME_SUPPORT:ALL_MAIN:2]
Ran a worldgen test and found something very interesting.
I found Axolotl tree cities popping up all over the ocean. (I've seen olm men civs pop up too in the ocean, but less so than axolotl men) I like to picture them having kelp forest cities in these case. No other animals had cities in ocean biomes, only on land biomes.
Fleeting Frames suggested to me that I test this by setting only ocean biomes to human civs. I did this. The game kept rejecting worlds at the civ placement stage over and over because it couldn't place humans in a suitable area. The game just up and told me it couldn't make worlds.
Though, bizarrely, I've seen Kea people with cities in the ocean in one worldgen instance. I assume this is a worldgen bug that happens when two civs go to war and the kea people happened to take over this specific city, but I'll have to do some more investigating in this matter.