If the body is still around, you can check the belly scales. They go all the way across back to the anus. Past there, if they go all the way across, it's a cottonmouth. If they are two scales that meet in the middle, it's a water snake.
Mr. nasty is a good name for a cottonmouth, they have a nasty, agressive disposition. I've had them try to get in the boat with me,:shock: a few years ago while walking the river bank, i looked up just in time to see one on a willow limb drawn back, waiting for me to take another step. Most times,even giving a chance to run (like most snakes) they will hold there ground. If you see a snake swimming in the water with his head standing up, it's a cottonmouth. Jack