Aquatic Creatures (anything that spawns in water) will not be able to spawn, if the bottom of the ocean is spawn proof. Meaning that by slabbing the ocean, you prevent all fish, squid, dolphin etc mobs from spawning in the water above.

This make sense for drowns, as they require the blocks to spawn on, so any spawn proofing would stop them from spawning. However fish, squid dolphins etc? Why should they check if the bottom of the ocean is spawn proof or not? They only need water to spawn.

How to reproduce:
1. Build a tank of water at world height, over an ocean biome. (aquatic things spawn at world height MCPE-44252)
2. Set up a repeating command to kill anything not around you, so things can spawn in your tank. `kill @e[type=!player,rm=30]`
3. Make the bottom of your tank out of stone, observe things spawning.
4. Place a layer of glass above the stone floor, kill everything in the tank, and nothing else will respawn.
5. You can replace that glass with slabs, pressure plates, signs, fences, anything you like that's not spawnable for aquatic things, and nothing will ever spawn in the tank.

This typically isn't an issue, until it comes to squid farms (see image below with fence gates below water). Having any form of water, with signs or fences below it, will never allow anything to spawn in the water. This breaks about 90% of squid farms and concepts immediately. This on top of all the other issues with squids and spawning, makes getting any amount of ink in a world very very tedious indeed.

For more info on this bug, and other issues with squids/aquatic creatures check out my recent bugrock of the week episode on this topic:
https://youtu.be/Qt7npxE9UzM