I was up late last night cracking this nut and I have a solution using volume color, altitude, and a volumetric slab. A cube could also be used if the camera is in tight and you don't see the edges of the cube but I chose a slab because it is an infinite volume - kinda like the ocean ; ) First of all you were on the right track using the altitude color, but you need to use it in the water - here's how. I'm going to upload a series of jpegs to explain it so hang on. -Scott