[jinx]

Originally Posted by capnhowdy

I'm fairly sure those are water moccasins. Keep your distance.

Their fat bodies give them away.

Is that a "dark stripe that runs behind the eye alongside the head"?

Some distinguishing features: link
The Cottonmouth has elliptical, cat-like pupils while non-venomous water snakes have round pupils. However, when a Cottonmouth's pupils are dilated, they're almost round, so be aware of that.

Cottonmouths have a dark stripe that runs behind the eye alongside the head, whereas harmless water snakes have other head patterns.

Cottonmouths tend (not always, but tend to) to swim high in the water, appearing somewhat overly buoyant. The harmless water snakes tend to swim mostly submerged.

Cottonmouths tend to retreat slowly, while water snakes tend to zip out of the way whenever possible. I know, however, that some harmless water snakes will stand their ground and act aggressive when disturbed.