I used to where knee braces for snocross, actually I used the evs visions... I havent used them in water. But I imagine they would hold up ok, or at least as good as they do out of water. The knee cup on mine started falling off after a few years... But that was with a ton of abuse. I think if you grease the metal joints or dry them very well, you will have no problems.
After breaking my femur right where the knee brace stopped... And realizing that a doc can fix a knee way better than a femur I just use KT tape and knee pads.
Have you tried KT tape? It stays on well in water for me, but it won't stop a hyperextension..