I have a 100 gal water trough for the livestock that I shoot into. That is plenty of water for any handgun. I've also shot into a swimming pool. Absolutely no problems doing so. Someone told me that show Myth Busters did a piece on water penetration, and the results were very surprising. Takes very little water to slow down a handgun bullet. I don't watch TV, so I've never seen it, but maybe someone here has.
It's great backyard fun, but has little practical self defense conclusions.