Cottonmouths are very misunderstood, as all snakes are. Cottonmouths have a reputation of being very aggressive. I have found that not true. Now I'm sure there are some very aggressive individuals, but none of the ones I have ever had contact with have been aggressive at all. There is an old saying about them that seems to be very true. First they are cowards. Next they are bluffers. Then they are warriors. If they know you're coming towards them they will head for the closest water and disappear. If you surprise them they will almost always stand their ground. They will use different tactics to scare you away. Hold their mouth open and show you their fangs is their first and favorite bluff. They will also expand their body to look bigger than they really are, and they will shake their tail on the ground to sound like a rattlesnake. I have never personally seen one get to the warrior phase. I have never even had a cottonmouth attempt to strike at me. I heard about a study that tested how much a cottonmouth would take before striking. Every cottonmouth tested would only strike if stepped on or restrained where it couldn't escape. Even if lightly handled they didn't strike. That don't sound aggressive to me at all, but defensive just the same as any other wild animal.
I have found that copperheads are slightly more aggressive that cottonmouths. Cottonmouths will use the bluffing tactic until they are left alone or pushed to the point that have to bite. Copperheads will shake it's tail and stay ready to strike. If you come towards it it will do a few bluff strikes even if you are way out of it's strike range. It'll just strike at the air. Then if you get close enough it will strike and make contact.