It is true that some species of water snake possess the ability to emit a fluid through their skin which will chemically react with some metals and can be dangerous, but it is also true that these snakes do not pose a serious threat to humans.
I would only worry about them if I were standing on or near them while they were performing this behavior. But in general, unless you are actively provoking the animal by standing on top of them while they curl up defensively, water snakes present very little danger to people.