Rebecca, this looks very much like a water moccasin to me. The triangular-shaped head and markings are very close to the pictures I have seen of a water moccasin (cottonmouth) and in real life. When we lived in Houston, a bayou ran right in our backyard, and we would fairly frequently see them, and as memory serves, this matches pretty closely. See this pic of a water moccasin here: http://www.critterclipart.com/images/... and compare.

I'm not sure when you took this picture... but, if you're still looking for help identifying this snake here's what I found out by doing a Google Image search for "Texas Water Snake". According to most "experts" on the internet; the vast majority of the snakes we will see in/around water here in STX are Nerodia rhombifer rhombifer or Common Diamond-back Water Snakes (non-poisonous). But, your snake could also be a Cottonmouth (very dangerous!!). The website "bugsinthenews.com" says that it's very difficult to tell the difference between the dangerous Cottonmouth and the harmless Nerodia when they're on land because they share similar coloration and shape. However, when a Cottonmouth is in the water its body floats and, when a Nerodia is in the water, its body will rest mostly/entirely under the surface. Personally, I admire and respect all snakes since they help to keep our pest populations under control and, I treat all snakes in the wild as though they are dangerous and give them a wide birth.
Hope this helps.
-Ralphy