Water snakes are any of a large number of snakes found throughout the world that swim and hunt in the water. Although most species are not poisonous, those that are include some of the most deadly snakes known. For this reason, care should be taken when faced with an unknown water snake.
Although all snakes can swim, some are particularly at home in the water. These snakes, which include cottonmouths, Northern water snakes, Asiatic water snakes, Rainbow water snakes, and mangrove snakes, can be very dangerous for unaware bathers or swimmers. At the same time, these snakes have nothing to propel off of, making their strikes much slower.
Most snakes, even water snakes, are simply not as effective in combat when not on land. For this reason, they will tend to not attack unless provoked, and for the most part, snakes in the water will run away rather than fight. That said, if a snake feels threatened or cornered, or is accidentally jarred, it will likely lash out.
The best thing to do to avoid undesired encounters with water snakes is to stay away from places where they like to sun themselves. Primarily, this means overhanging tree branches. Many snakes sit the branches to gather sunlight, and then drop down into the water if they detect motion. They’re not dropping to attack — they’re actually trying to run away — but if they happen to land in a boat or on a swimmer, they can panic and attack.
In North America, only one species is poisonous: the cottonmouth water moccasin. Water moccasins are related to rattlesnakes and copperheads, and can be deadly. They have trapjaws that can snap shut quickly and with great force, driving its venomous fangs deep into flesh. Their name derives from their habit of lying with their mouths open, showing the white interiors. Although often maligned as being aggressive, water moccasins are actually fairly docile. Generally, they will avoid humans, unless provoked in some way.
A number of non-poisonous water snakes live in North America as well. These include the Florida green water snake, the diamondback water snake, the brown water snake, the salt marsh snake, the plainbelly water snake, and the southern banded water snake. None of these snakes are aggressive or poisonous, but as a number of them are at least somewhat similar in appearance to water moccasins, care should be taken anyway, unless identification is certain.
In South America, one of the most common water snakes is the mangrove snake from Venezuela and the islands of Trinidad, Tobago, and the Guianas. Although not poisonous, they are fairly easily provoked, and their bite is painful and can cause infection. The Asiatic water snake, found throughout South-East Asia, China, Indonesia, and parts of Western Asia, is another non-venomous water snake. It is quite docile, and will rarely attack unless extremely provoked.