Cottonmouth

Cottonmouths are VENOMOUS, so do not handle unless you have proper training and/or experience with venomous snakes. Although they are venomous, after 10+ years of experience handling these snakes, I will say that they are generally NOT as aggressive as most people say they are. When harassed, they WILL defend themselves, typically by coiling in a defensive posture, and gaping (opening their mouths to show the white interior). When encountered on the move, these snakes remember where escape routes are located and will try to go towards those routes, so if you are in the way, that may lead to the misconception that they are "chasing" you. However, they are not. They are trying to escape from you, as you are a much bigger predator than they are.
That said, cottonmouths are easily confused with water snakes (which I think are more aggressive). They both are typically dark snakes, with some adults showing little or no pattern. Younger cottonmouths will have geometric patterns of lighter and darker color, ranging from dark charcoal gray to an almost pinkish red. Juveniles are characterized by their yellow-lime tails. Cottonmouths always have angular heads, a dark stripe down the face, and usually are fat-bodied.