How do i make my ocean biome super deep?

so my ocean biome when i make it can get down to y=32 or so just like normal deep oceans with a biome base height at -1.9 but its the same all through -1.5 through -1.9. once -2 though everything goes straight to bedrock and biome borders are straight down. i want an ocean biome that goes down to y=15 so i can add a large underwater structure but i cant figure it out. i've made 20 worlds all testing different base height but they are always at 32 or 0