How to make a biome generate as an ocean?

I am trying to make a lava ocean biome, but I can't seem to find how to do this. I tried to copy the nether oceans, but they don't seem to be biomes. I tried to copy the noise settings for an overworld ocean, but it generates at the terrain level of the surrounding biomes and does not have lava, even when below sea level. I've set the dimensions default fluid to lava but it doesn't change much.