Can't tell definitively from the photo. It does look to have a triangular shaped head larger than its neck and pits between its nostril and eyes so I would categorize it as a pit viper. Bull snakes I've seen have a head about the same size as its body width. As I stated above, this seems but only seems to be evident in your picture. If it coiled to attack and opened its mouth for you to see a milky white coloring then its a water moccasin/cotten mouth. Also , when they swim, pit vipers float entirely on the surface. If it is a cotton mouth, its one big one. Largest cotton mouth I've ever seen was about 4'. Patterns and coloration change from one snake to the next even amongst the same species so any color or pattern is not a safe way to judge a snake more so than its conformation