Mob grinder not working in Ocean/deep Ocean y230+

I made it in a deep Ocean biome , and none has spawned, their is 4 layers of 200 spawn-able snow blocks and I afk'd their for 20 mins, and absolutely none has spawned, I checked my f3 menu, looked for hostile mobs, and Have not seen any in the mob grinder, looked around the area for ton of hostiles, and none on the ground or in unlit caves.