I've waded chest deep water, in just shorts and tennis shoes, stood a few minutes to get that cast in just the right spot, I have had cottonmouths swim just a few feet from me, see me, then dive under water. I just keep fishing.
I will say this, if I had seen it first, I'm not sure if I would have went in right away.
Copperheads float too, I once saw one fall off a bluff, into the water. I don't think it was part of the plan, it got out asap. I was told as a child that all poisonous snakes float (most of the body) when in water. I have not seen all poisonous snakes so don't know that for sure.