Not to mention many water moccasins are nothing more than harmless nonpoisonous water snakes.
Nope, water snakes and Moccs are totally different, the problem comes from folks calling any darkish water snake a Moccasin. This is a matter of intellectual sloth, soon cured when a largish Moccassin gets really upset with you, and runs your butt up a tree. "Moccasin" relates to Agkistrodon piscivorus, and closely related pit vipers.
The Copperhead is a close cousin, not nearly so much aquatic, less venomous. As a Cottonmouth ages, the patterns and colors fade away into solid black, except for some highlights. They get massive in body diameter and spectacular in attitude. A Rattler may retreat,most Moccs don't know how.