I originally suggested this for Sodium (CaffeineMC/sodium-fabric#2472), but it ended up being rejected, so it might fit better here.

Basically, the idea is: if a block with a full bottom face exists above water or lava, then the fluid height would raise such that it would be coplanar with the underside of said block. Here's a comparison:

Since this would end up with the face 100% obscured if the block above is opaque, we could then cull it. This would eliminate lots of unneeded geometry in the Nether and above water caves, and things would also look a lot nicer from above and within the fluid as well (refer to the linked ticket for examples).

The fluid height would still snap up to full transparent blocks such as glass, but would not cull.

Thats not a terrible idea. It's a little complicated, since it would require me to change the hardcoded geometry.
Not sure if I have the time to do it either, so we will see.