It really isn't that hard to tell the difference in the water. Watch the shape of the body, the body in the water (is it "on the surface" or under the water), and where is the head held (up out of the water or pretty much on the surface). If the body is on the surface and the held is held out of the water, it's typically a water moccasin. If the body seem to be under the water surface, and head is barely out of the water, it's a harmless water snake.
If you have the habitat, you will have the snakes.
I did get a pretty good chuckle out of the cottonmouth or water moccasin comment.
Honestly, almost 90% of the water moccasin calls I get end up being diamondback water snakes or blotched water snakes as Log mentioned.