Back in my Bryce days (ahhh…) I used to do effective underwater scenes using coloured fog and a water texture (the sort of rippling surface in sunlight pattern similar to Hockney paintings, perhaps that is on the surface of your current image). You may need to adjust the image to make it match your fog colour and saturation.
Apply this texture to everything, or project it from a light. Be aware this light is coming from above only, and the light underwater is very diffuse. Shadows are deep and very soft.
stigWeard