Oceans all have stone floors. Apparently, this was also a Java bug.
Magma Blocks doesn't generate on the floor of aquifers and oceans
Ocean is a Cave
Minecraft Bedrock Betas Ocean Are Broken
Sand and gravel don't generate on the seabed. Песок и гравий не генерируются на дне океана
Sand and Gravel is not generated in Seafloors, stone is generating in seafloors
Cave generation causes seabed in oceans to merge
Ocean floors are made entirely out of stone related items
Ocean floor are now caves in experimental
warm ocean generating a stone floor instead of sand
let's see, that sand and gravel, etc; cease to exist in oceanic biomes after enabling experimental gameplay.
Oceans generate with stone floors