I've never actually heard of a major accident that wasn't caused by human error. Of course having water in your system does add a certain degree of risk, but if you set it up properly the worst you'll have is a drip (which you'll almost always catch before it does damage).
If you're really concerned, and looking into water cooling, there are non-conductive cooling fluids that you can buy to fill your system. From what I've read nothing really cools as efficiently as distilled water with a little bit of additive to keep algae from growing, but there are safety and maintenance benefits to using cooling fluids.
As I have not personally seen one I can't really say much. Personally I wouldn't use it for the reason that you alluded to. The dangers obviously are water based short circuits resulting in burn out of that circuit if nothing else. In addition the board itself is partially water based, long term exposure to moisture, even if not a short cir. will deterioate the MOBO. Best advice is to do a lot of head scratchin before plunking your $$ down.
Just in case you were asking about physical danger to yourself... It's not likely.. the fuses/breakers in the house will blow before you could shock yourself by having a water cooled machine. If a water leak were to happen and contact anything that "could" shock you, it would short out the machine and blow a breaker, or pop the power supply's fuse before it could get to you.