Be really careful using short cuts when identifying snakes as venomous or harmless. You'll hear that round pupils and bars on the jaws denote harmless water snakes as opposed to venomous cottonmouths. Following those characteristics, this photo is easily identified:
Except that its a venomous Florida cottonmouth (same traits are true for the Northern cottonmouth found in Texas). While the harmless water snakes will not have vertical slits for pupils, light conditions effect the shape of pupils - in low light cottonmouths can have round pupils. The bars on the jaw are not like the local water snakes which are on both the upper and lower jaws, however, when people give shortcuts, they often just say bars on the jaw.
This picture does show the hooded eyes which are mentioned upthread. From the top, the eyes are not visible. Water snakes do not have hooded eyes and their eyes are more on top as shown in @BearSchlong's picture above.
The best rule is that if a short cut is not 100% in every situation, its not a good short cut.