Making a new ocean biome with custom depth. (1.20)

I'm trying to make a new ocean biome ("deep_ocean") which generates alongside the normal ones, except with a far deeper ocean floor. Ideally down to almost bedrock. 
I've seen a few datapacks out there that lower the entire ocean floor, but I can't find any that just make one ocean biome deeper.
I've made a fair few datapacks before but in all honesty I have no idea how the new terrain generation system works.