Cottonmouth or water snake
Thank you for your question. The snake in your photographs is a Brown Watersnake, scientific name (Nerodia taxispilota). It is a nonvenomous species. It would likely strike to defend itself if cornered by a dog or cat, but this snake is in more danger from your animals than they are from it. Like other species of watersnakes, it is often mistaken for the venomous cottonmouth (Agkistrodon piscivorus), whose distribution often overlaps those of some of the watersnake species. Brown watersnakes are found from southeastern Virginia, through most of the coastal plains of the Carolinas, south to Florida and west to Alabama. They are found in large rivers and associated swamps, as well as reservoirs created by damming rivers and streams. They can be found in other aquatic habitats, but not as often.
The brown watersnake is one of the larger species of watersnakes. Typical length is a little over 3 feet, but a maximum length of approximately 6 feet is possible. This is one of the heavier bodied watersnakes, and because older females can often lose their pattern, as seen in your photographs, and become brownish grey or mud colored this leads to them being confused with the cottonmouth.
Brownsnakes feed primarily on fish, especially catfish.
This species is usually not found far from water. If threatened, it immediately attempts to escape to water. This snake often basks in low hanging branches over water, where, if threatened, it simply drops into the water. If captured, however, this snake will release copious amounts of strong-smelling musk from its anal glands and bite you repeatedly until you let them go. That being said, you have to be pretty fast to catch one.
Your location didn't come through with your question, but here are a couple of excellent field guides that can be useful in identifying snakes depending on where you live:
Gibbons, Whit. (2017). Snakes Of The Eastern United States. Athens: The University of Georgia Press.
Gibbons, Whit and Mike Dorcas. (2015). Snakes of the Southeast. 2nd edition. Athens: The University of Georgia Press.
I hope this answers your question. Great photos by the way. Thank you for contacting Ask an Expert.