If you care to check my image "Do Androids brag about the one that got away", I created the two portions seperately and composited them in photoshop. That way I could get the right atmosphere I wanted in each of the two medium. I feathered the edge of the water and along it I painted in with a brush set at dissolve (50%) which I motion blurred at an angle to give the impression of light rays. This is of course one of the methods. There are others as well.