Also, if you're trying to convince the viewer that we are looking through glass, instead of BEING under/in the water with the swimmer, then you should do something about that.

Put a big square between the camera and the figure. Then you need to think about what you want to reflect in that square. I'd surround the whole scene with my environment sphere, and I'd set up some panoramic HDR photo of an interior on the environment sphere. Then I'd set up the square to reflect the environment.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)