The atmospheric/volumetric effects don't seem to fit with this problem, since they need to be confined by the water surface. But the Fresnel node may be useful. I think you have to go back to P4 to get an answer: not a tool in the program but the methods people used for mist and fog effects. Apply several almost-parallel planes below the water surface, at increasing depth. Make them mostly transparent with a noise variation and some colour tint. I think Anthony Appleyard put something like this in freestuff, for underwater scenes. you might have to blur the texture you're going to apply to the underwater terrain, and I'd be careful about objects which intersect these planes. P5/6 have things like wave deformers which might help with this sort of thing. DAZ 3D and Runtime DNA have both sold underwater stuff, and there might be discussions archived in their forums.