How would I create flooded terrain?

I have a custom dimension using terrain generation similar to the overworld's, with the main catch being that underneath the surface is entirely air.

To get this to work, I had to make a custom air block and use it as the "main filler block": This produces results that are essentially a hollow world. There's thin layer of solid terrain with nothing underneath it until you reach bedrock.

However, I want to attempt to replace this empty layer with water, essentially creating endless oceans underneath the thin layer of solid terrain. I've tried using water as the

main filler block, and using procedures to replace my custom air block with it, but neither have worked. Any help is welcome.