I'd always pondered some terrible way to combine my two favorite hobbies: scuba and climbing. The only scenario I could ever come up with is to do some sort of inverted climbing underwater. The idea being you inflate your BC (buoyancy compensator) so that you're positive, and then climb upside down on a wall (underwater, of course). The risk then is not falling downward, but ripping off and suffering some kind of horrible over-expansion injury or having an out of control ascent and getting bent. I've never given it a try, don't really have a desire to. You could definitely come up with some cool protection methods though.