Aside from the water everywhere, is there anything preventing mob spawns in Ocean Biomes, like there is for Mushroom biomes?
No, there isn't. Mushroom biomes are the only biome which actually prevent mob spawning. Sometimes in an ocean biome you will find mines or ravines that breach the ocean floor and spot mobs that have swam up from there. Any small islands will also produce mobs. Any structures you build there which aren't lit and aren't made of transparent blocks can also spawn mobs.
I played on a server that had many parts of the map explored before the biomes update, I've had to herd animals to my place on the map since for whatever reason it shows up as an ocean biome in the debugger. There are plenty of hills, grass, trees and light in my region, but no passive mobs ever spawn randomly (Squid and aggressive mobs are a different story).
I had to look a while on the wiki to finally figure out what was going on.
Animals do not spawn in desert or ocean biomes, with the exception of squid.