What I’m trying to do is easier to understand if I explain it like this:
Imagine a giant cube that’s a lake of static water. At the top there’s a character. The deeper you put character in the cube, the darker he gets, gradually along his body.
or: imagine a lake with dirty water. The deeper you put the object, the harder it is to see it.
It’s possible to do it in Blender? Maybe with a gradient texture (where the bottom is solid black and the top transparent) or maybe fog/mist just inside the cube?