Check this out. This is simulating looking from something like water out into something like air. At the surface, rays that are going into it fairly directly are sent through, but with a huge bend, creating a fisheye effect. Rays that hit the surface at more of a glancing angles refract in such a way that they reflect back into the material. This is now simulated perfectly.

This image perfectly shows off what I've simulated. There is a clear, well defined boundary where the camera can see through the water and what's on the other side, and where it can't and just reflects the swimmer.