Practically anywhere in the southeastern United States – as far north as Indiana and as far west as Texas – the snake swimming toward your boat is as likely a venomous water moccasin (Agkistrodon piscivorus), and is a harmless water snake. Water moccasins are pit vipers, which means they have large, heavy bodies and triangular heads. At least one other snake simulates these characteristics, though, so you need more information to make a positive identification. Fortunately, water moccasins have idiosyncratic markings and swimming habits, so while identifying one in a fit of panic may not be easy, it's doable.
Water Moccasin Physical Appearance
A water moccasin may appear uniformly dark brown or black at first, but if you look closely, you can often distinguish light brown and yellowish bands encircling its heavily scaled body. If the snake is young enough, these markings may be bright. Although they aren't diamond-shaped, the bands are somewhat reminiscent of the markings on a rattlesnake, which makes sense because the rattlesnake is a relative.
Like all pit vipers, the water moccasin has a neck that is much narrower than its triangular head and stout body. You probably won't want to get close enough to note this, but a water moccasin has vertical pupils shaped like slits, rather than the round pupils of most harmless water snakes. It also has a single row of scales on the tail, unlike non-venomous snakes, which have two rows side-by-side.
Cottonmouths Are Water Moccasins
The water moccasin is also known as the cottonmouth, and the reason comes from the defensive posture the snake assumes when threatened. It coils its body, raises its head and opens its mouth as wide as it can. The color of the skin inside the snake's mouth is as white as cotton – hence the name cottonmouth. If you see this behavior, it's time to back off gently but quickly, because the snake is ready to strike.
Water Moccasins Love Water
You won't see water moccasins far from water. They prefer ponds, lakes and streams with abundant food for them to catch. Cottonmouths eat fish, amphibians, birds, mammals, baby alligators and smaller cottonmouths.
A swimming cottonmouth is easy to distinguish from a common water snake. It keeps most of its body above the water, almost as if it is floating. Water snakes, on the other hand, keep most of their bodies below the water; only the head is visible.
When not swimming, water moccasins like to soak up the sun on rocks and logs near the water's edge. They don't climb trees, so you don't have to worry about having one drop on your head, but if you're walking by a stream or lake – even in winter – be sure to check the far side of a log before stepping over it.
Beware of Imitations
The banded water snake (Nerodia fasciata) mimics characteristics of the water moccasin to enjoy the benefits of a venom delivery system without actually having one of its own. It flattens its head and body when threatened to give a more-than-passable rendition of a water moccasin's fat body and triangular head. It isn't a perfect impression, though. It's belied by the water snake's too-slender torso, extra-long, narrow tail and markings that don't fade into black toward the tail the way a water moccasin's markings do.
Even when it's not trying, the banded water snake looks similar to a water moccasin, but the most telltale difference between them is the heat-sensing pit that gives pit vipers their name. It's located on the forehead just above and between the nostrils of the water moccasin. The banded water snake has no such pit.