One would expect that transparent surfaces will support specular lighting without any problems. Unfortunately I’m afraid that UE doesn’t allow that right now. I tried to port my ocean shader from UE3, and had big problems getting specular highlights on water surface. I checked how Epic created water in Cave example scene. Turns out they used two plane meshes one on top of other. Lower plane simulated water color and specular reflections, while top plane simulated water refraction. Guess we will need to wait until this gets improved.