I think I can answer this one -- I'm usually pretty good at these riddles. The bartender was an old codger with a hearing problem. When the man asked for a glass of water, the bartender mistakenly thought he said, "Excuse me, but would you pull out a gun and point it at me please."
The bartender, being a nice guy, simply honored the man's request.
It's really not that difficult of a riddle.
If you want to hang around this thread...try and come up with something a little tougher than that.

Haha just kidding, I pulled that from another forum as I went to cheat and look for the answer, which I won't post.