Everyone who frequents the outdoors in MS knows about those badass cottonmouths. Sometimes they avoid you, but just as often they'll stand their ground or even attack, without provocation. Once while shotgunning for early-season teal, I had to shoot a big one to keep it out of my waders. They will also try to get in a boat at times, and most swamp fishermen carry a pistol, just in case.
I was fishing around the bank of Bluff Lake near Starkville when in college, which is a Federal refuge and no guns are allowed on the lake. It was full of big cottonmouths, and you had to be constantly on guard. I killed four that day with my walking stick, and didn't go back.
You can tell them from the harmless water snakes when they're swimming because the heavy body of the cottonmouth sinks. Only the head shows above water. I accidentally hooked one behind the head one day with a fly rod, because I thought it was a frog and there might be a fish behind it. Yes, that was exciting!