The northern water snake (Nerodia) is a native of the eastern U.S. and southern Canada. It grows to about three feet long, and is colored in a brown and tan pattern. Water snakes are somewhat aggressive and may bite when handled, but can be calmed down in time. They should probably not be someone's first snake.
A twenty gallon aquarium or other container about that size can be used as a cage, but water snakes will appreciate having more room than that. The cage needs to have a tight fitting, escape proof lid. Screen is a good material for the lid because it allows ventilation. Check at the pet store for clips to keep the lid secure.
Despite the name "water snake," these snakes can suffer from skin infections and blisters if the enclosure is too damp. If the snake develops water blisters from too much wetness in the environment, remove the large swimming pool and replace it with a small water dish for a while. Like other snakes, water snakes are escape artists, so the lid needs to be held on with clips, straps, or some other method.
Newspaper makes a good substrate for these snakes because it is easy to replace when soiled. Water snakes have loose droppings that can be difficult to clean with bedding material, but aspen shavings can be used. Don't use cedar or pine shavings because they are toxic to snakes. Some people like to combine half playground sand with half potting soil for a natural substrate.
The enclosure should include clean water at all times. Ideally the dish should be large enough for the snake to swim in. It should also be hard to tip. Dog water dishes often meet these requirements and can be used in the enclosure. A branch or two for climbing are another nice addition. Whatever "furnishings" you use in a snake enclosure will have to be cleaned and disinfected monthly, so keep that in mind when setting up the cage. Some people like to add plastic plants to make the enclosure more natural looking. They will have to be cleaned, too, to maintain the health of the snake.
Places to hide are important to the security of any snake. These can be as simple as a cereal box on its side with the lid cut off or as elaborate as a plastic hide box from the pet store. Upside down butter dishes with an entry hole cut in the side are a cheap option. Snakes tend to like the fit to be a little snug, and will coil up small when hiding. Keep that in mind when choosing hides.
Because they are native to North America, water snakes don't need much heating in their enclosures but do need to be kept from cold temperatures. It is good to provide them with a temperature gradient of 70 degrees Fahrenheit (21.1 degrees Celsius) on the cool end to 84 degrees in a basking spot on the other end. At night, let the warmer end cool down.
If the enclosure needs a little outside help to keep this temperature, a simple low wattage light bulb over the warm end can often provide all the extra warmth needed. Use thermometers to make sure the enclosure is within the recommended temperature range. An undertank heating pad is also a good plan. Use it only under the warm end, and monitor the temperature frequently. It may need a thermostat or rheostat attached in order to keep it from getting too warm.
Water snakes will happily live on a diet of earthworms and small fish. They also like frogs and salamanders if you can get them. Feeding them aquatic animals can be accomplished by placing a tub of water in the enclosure and letting the snake swim around catching the fish. Water snakes will eat pinky mice and rats, too, and can be trained to eat them pre-killed. If they are eating mice, one meal a week is plenty, but twice a week is better for snakes that are eating a lot of fish and amphibians.
Frozen mice are often the most convenient diet for a captive snake. Use a mouse that is smaller than the girth of the snake. Always thaw the mouse first. If the snake is resistant to eating a pre-killed mouse, you can try getting the scent of a frog, toad, or fish on it to make it more desirable. Water snakes prefer pinkies over older mice. In the wild they do not eat prey that has fur, so they may have a natural aversion to these older animals.
It is thought that fish alone are an inadequate diet for a water snake, and that they will do better with a varied diet. Feed snakes individually or they will fight over the food. The eating response is instinctive, and they can even try to eat each other when food is present.
Regular cage cleaning helps to prevent problems like ticks, mites, and scale rot. A good solution for disinfecting the enclosure is one ounce of household bleach in ten ounces of water. You should not use cleaners like Lysol because they can kill your snake. After cleaning with the bleach solution, the cage should be rinsed and dried thoroughly, and new substrate should be added. Water snakes need to be able to get dry even though they like soaking and swimming. Make sure the cage is dry outside of the water bowl.
Water snakes can become tame when handled, but will usually object and even bite when it is first tried. Stick to a schedule of short periods of handling, like 5 to 10 minutes. Never handle a snake within 48 hours of feeding. Also, leave them alone during shedding.
Water snakes are not often found in pet stores, and are usually wild caught even if they are. A herpetology show might provide information about where you can buy one of these snakes. While their feeding and care requirements can be somewhat labor intensive, they make an interesting pet for a lot of people.