Ohiotuber,
If you are fishing trout water, then it is cool enough that I wouldn't worry too much about cottonmouths. I have never encountered rattlers in water, although some friends of mine might have, maybe. Copperheads are another issue in cool streams. In warm water, anything goes. I have great hopes that neoprene will give me an advantage now. I can tell you that seeing a big cottonmouth swimming straight for your zipper is a VERY unwelcomed sight. Luckily I had a friend in a canoe nearby. He became a much better friend, that day! The snakes that have bitten me underwater have all been non-poisonous but there used to be reports of people being bitten underwater by rattlesnakes and cottonmouths. Too many people have seen a cottonmouth surface with a fish in its jaws for them NOT to be able to bite underwater. Striking is different, but biting doesn't seem to be a big deal to submerged snakes. My hope for waders is that I will be able to pull off a snake that is trying to bite me before it can get a good grip with its jaws. Occasionally snakes will take a lure that was running submerged. Last time I went out fishin on the Little Harpeth I had to move to get away from a non-poisonous, water snake that was too interested in my lure. If you are going to be in a boat or a canoe, then remember that snakes DO climb trees and bushes and DO drop down when startled. Sometimes they land in the boat with you. Not a lot of fun.
Ed