Impressive backlit images do not necessarily require the use of a flash. Just aim towards the surface and use the sun. Often, that is all there is to it. If you understand how to get the exposure right, that is.
Under most circumstances, underwater photographers balance light from flash with ambient light to create a certain ambiance by adding illumination to the front, above, below or the sides of the subject. But there are a range of subjects that only work if they are taken against the light.