The Northern Watersnake (Nerodia sipedon sipedon) may well be the most misidentified snake in the East. Due to its living in close proximity to water, large size, variable coloration, ability to flatten its heads to appear triangular, and what some consider a feisty nature, many people confuse it for the venomous Water Moccasin (Cottonmouth) snake. This despite cottonmouths actually not living in the DC area, being limited to Southern Virginia at most.
Northern Watersnakes are thick-bodied snakes, up to 55 inches in length (though most are 2-3 feet on average). They can have a wide range of color, and the tannins and mud in the water may also stain them. Most however have dark bands across their back that are wider than the light colored bands along their bodies. The dark bands narrow along their sides and thicken along the top of their backs. These bands tend to turn into blotches by the time they reach their tails. The bands also fade as they grow older, becoming difficult to see on the oldest and largest specimens. These snakes also have crescent-shaped marks along their bellies, but that is obviously not an easy thing to see.
Watersnakes (all one word is the scientifically accepted way of spelling the name) are quite common, being found in every county in Virginia and almost all the DC area. They are almost always found in close proximity to water. They can swim completely submerged, which differs from many other types of snakes.