Ooh a subject I know a bit about. Inserting random information below!
There are 4 types of poisonous snakes in the USA. (not including the different ones in each species, IE Timber Rattler, Western Diamondback rattler)
Cottonmouths, Copperheads, Rattlesnakes, and Coral Snakes.
Of the 4, three are Pit Vipers which allow them to sense prey with heat pits located to each side of its face. The 4th is a coral snake which is a tiny cousin to the cobra (similar venom) but they are the least dangerous because of their size, number, regions, likelihood of being bitten.
Of the 3 Pit Vipers...
Rattlesnakes dont tend to be found swimming much (I am guessing their tails dont do well in long submersion) but they do like to be near water.
Copperheads wouldnt avoid the water but still wouldnt be that common, they like grassy/bushy areas like fields of hay, blackberry bushes (which mice like) etc.
Cottonmouth's AKA Water Moccassin AKA Blacksnakes are the snakes that tend to live right on the banks of rivers, lakes, ponds etc. They usually put out a fairly musky odor when you get near enough. They are generally dark enough that you cannot distinguish a pattern but I have seen them in a light grey color. They tend to swim differently than most other water snakes. Most snakes swim body down head up, Cottonmouths usually swim with their whole bodies on top of the water. They can also strike underneath the water which is how they get the fish. Generally they are very curious and often scare the bejeezus out of boaters when they approach.
Chance of being bitten is not very high if you leave them alone, but they may come near you anyways. If you are bitten of course seek medical help (no tourniquet's, cutting and sucking, etc) Chance of death from one of these bites is also very low, higher if very old or very young.
Most snakes with venom inject very little of it in the bite (maybe 10%) A few are known to be a bit nastier like the Western Diamondback rattlesnake which can inject almost all of it (90%) in a single bite when provoked. Where others, like the Timber rattlesnake are known to be fairly docile and likely wont strike unless stepped on. (exceptions to the rule abound im sure)
I am not an expert but I have always had a strong interest, for those who have seen Lonesome Dove just be careful crossing rivers on horseback after a storm.
Pit Viper venom is fairly mild in comparison to the horrible types some other countries deal with.
Identification - Pit Vipers (Rattlesnakes, Cottonmouth, Copperheads) tend to have a fatter body than most snakes proportionally, they are the only US snakes with eliptical eyes (like a cat) They also tend to have a triangular shaped head.
The coral snake is the only poisonous US snake without eliptical eyes, its identified by its bands of color. Red stripes over Yellow seperated by black. Other snakes have similar colors in the US but the patterns are different. There is a saying ive heard several ways.. Red on black, friend to Jack. Red on Yellow, kills a fellow.
-Josh