Short heavy bodied pit viper; head wider than the neck and mouth white inside. These snakes range in color from mostly dark brown to having broad yellow-brown bands. They tend to swim with most of their body on top of the water and visible unlike other non-venomous snakes that swim with most of their body below the surface of the water.
Freshwater habitats of all kinds. The world's only semi-aquatic viper, its a strong swimmer and can tolerate saltwater.
I've encountered several water moccasins and none of them have been aggressive. My experience has been that they do not move out of your way but may give enough movement to warn of their presence. When hiking through wetlands, its up to you to be observant and avoid stepping too close to these guys. They have excellent camouflage.