I expected to get a world that was cold ocean only.

But what happened was that the world generated as a plains biome with underwater structures on land. Looking in f3 however says that everything is cold ocean. All spaces that would have been open (underneath y63) if it had been a plains biome, were filled with water and generated as a cold ocean. I've only tested this/been affected by this in 1.19.2.

Steps to Reproduce:
1. Go to "Create New World"
2. Set world type to "Single Biome" and choose "Cold Ocean"
3. Create the world.