The markings don't look like any of the water snakes I'm familiar with.
Water Snake pictures
It's probably a rat snake of some kind. You didn't say how big it was. The juvenile markings of some rat snakes look similar to the snake in your photo. But the head in your photo doesn't look like a juvenile, in which the head is slightly out-of-proportion-larger, so to speak... it could be a bit beyond that age, however.
http://farm4.static.flickr.com/3131/3175592678_573f8420eb.jpg
Details
http://img2.allposters.com/images/PTGPOD/OSROA-00000300-001-FB.jpg
It's definitely not a venomous snake unless it's some funky viper somebody brought home from some foreign country and released.