Not that copperheads can't be in water.
But, if it was in the water, 99% chance they are all water snakes.
Water snakes are much more common, much more likely to be seen, and much more likely to be in water.
If you do see a copperhead swimming in deeper water, it will generally float. The whole body basically above water from head to tail. Whereas a water snake will be mostly submerged, with just the head and neck area above water, and sometimes fully under.
Here's some pictures for comparison...
http://www.lifeandscience.org/nature-watch-blog/posts/copperhead-vs-northern-water-snake
Water snakes have a nasty disposition. Perhaps the nastiest of snakes I've come across. But they are not venomous.