My water dimension has pockets of air (2025.3)

Hello, I would like to know how I could make my world completely underwater.

Now, I've managed to make a custom dimension with one single ocean biome, and for the most part, it's going well, except for one problem.

There's massive pockets of air in my dimension, and even the occasional lava lake you would see in a cave.

How could I fix this issue?