I think it depends on the exact type of brace.
I wear a leg brace, and my technician has said that I can wear it at the water parks and the beach as long as I am very careful to rinse it in fresh water regularly so that salt and chlorine do not have a chance to dry in it's joints, and clean it *very* *very* throroughly *immediately* after. He did say that if I do it more than a few times it will make the pads deterioriate much faster.
I wasn't using the brace last time we did Typhoon Lagoon, but I have found some other waterparks won't let me on most of the rides with the brace. They say it's an insurance issue.