I was fishing in our stock pond one spring many years ago. The overflow spread out about 50 feet but was only and inch or two deep. I was looking for high spots, trying to keep my feet dry. Had made it about 20 feet when I heard splashing behind me. Looked back and saw a cottonmouth coming toward me. I no longer looked for high spots and hightailed it out of there. Another time when the water was much lower I was fishing from a spot that was usually under water. It was a small spit with water on both sides. A moccasin poked its head up out of the murky water about 6 feet away. After a couple of seconds it went down and then came up about 3 feet away for less than a second. When it went down again I cleared out.
I have killed copperheads with an axe iin my youth but consider a shotgun the best medicine for a cottonmouth. Have never seen a live rattlesnake but there are a few in the state.