I prefer a two render approach to doing underwater scenes. The first render contains all the objects, terrains, plants, etc... I stick that render on a 2d plane, then using spheres and cones I create a second render with water murkiness effect. I have a sample in my gallery if you want to see. I went with a greenish tint just to be artsy. I could have easily went with a more natural color. Hope that helps and good luck!