Attached Link: https://en.wikipedia.org/wiki/Bow_thruster

This image is a Poser render of my surface (and short-dive-submersible) grab-dredger. The sea surface is the ground plane re-colored and 50% transparent (edge transparency = 0%); the deep sea beyond visibility limit is the background with color red=0, green=128, blue=255. How could I make the effect of underwater visibility decreasing with depth, while keeping the air above water level clear?

(The big hole each side in its bows is the inlet and exit of a bow thruster)