As it stands 1.21.1 Single Biome Generation does not generate just the biome you wish to generate.

As this is a ticket that is supposedly resolved and "works as intended" I am going to provide screenshots of that not being the case.

The image labeled as MC1 shows the generation seed set to 1. The actual generation setting for the single biome as I want it is set to a "Deep Lukewarm Ocean".

The Image labeled as MC2 is showing the actual generated landmass however. Including underwater ruins.

The Third image labeled as MC3 is showing the in world seed as it was generated using the settings as specified above.

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This is a bug or issue or whatever it may be that has been a thing for a long time now. It is not working as intended. This is not a single ocean biome. Its an ocean thats not an ocean. Its a plains biome pretending to be an ocean.

I do apologize if my tone is rude. But this has been plaguing several versions and updates and has never been fixed. We want to use single ocean biome worlds where the ocean, the water of it is the only thing covering the world but are denied such by a simple "Was already resolved" when that simply is not the case.