I was testing the new vesion of minecraft when i found creepers, bats, enderman, etc. all spawning at the bottom of rivers and oceans
I have attached two pictures of the mobs under the water i found the enderman one partically interesting since he wasn't teleporting like crazy while in the water or taking any damage.
The reason this is, is because of the kelp the mobs spawn in the kelp and the water doesn't affect them while there in the same block as the kelp.

squids also that swim into the kelp blocks will sink to the bottom then drown.