I attempted to create a custom world that has larger oceans by altering the "continentalness" noise setting for all biomes. I did this with software, by altering the continentalness of all biomes with a simple function f(x) = -0.3x² + x + 0.3 for values between -1 and 1 and leaving values outside these ranges unaltered. (The function maps -1 to -1, +1 to +1 and shifts the location of intermediate values towards the positive direction so that shores are moved from -0.15 to near +0.15).

When I did this, the ocean biomes were enlarged as expected. However, the terrain was not changed. This creates large expanses of ocean biomes that are above sea level.

I used a world generation data pack that can be found here: https://drive.google.com/file/d/1OyLG704MKEILKQGY9utUreFms7G7v2jw/view?usp=sharing
I substituted the "Overworld" biome settings in "data/minecraft/dimension" with the attached "overworld.json".
The result were the attached biome map "oceans_biome.png" and world map "oceans_world.jpg".

Note: The "overworld.json" has wonky whitespace, which I intend to fix later. However, the placement of whitespace between tokens isn't important in JSON files.