I do love the concept of expanding upon the underwater biomes! This is perhaps one of the ideas I'd like to most see in the game, since it just builds upon what we already have without changing too much.
It feels rewarding once you finally get a swimming creature with gills, underwater breathing and such, but it quickly can get repetitive with traveling circles around an island and just catching fish/digging. More underwater predators, perhaps in deeper waters, and underwater nests are wonderful ideas.