I agree that doing it with a volumetric cube can be a toughie. Thats why I took the easy way out I guess...I did 2 seperate images! I composed the two images seperately (as I used the fog and haze settings to simulate murky river conditions. I kept the camera on water level in the "above water portion". I then deleted all the content which I didn't need "under water", i.e. I kept only the rocks and the fishing line (as I needed their reference under water), tilted the camera down and imported in the underwater scene and changed the sky setting to underwater (which I had saved). After rendering, I took the two images and cut out the above water portion along the water ripples and feathered the edges. I then drew a white line set to dissolve 40-50% along the edge on a seperate layer and then motion blurred it along the direction of light...this made it into bit of sparkles along the water edge. The fish line and rocks acted as continuity and the image set in perfect! I'm reffering to my"Do androids brag about the one that got away!". Check if this works for you?