My first thought was cottonmouth but like Dave said, many snakes have markings that make them look like more dangerous snakes as a defense against predation.
Cottonmouths contribute to the problem by being so variable in color, themselves.
I've learned a little bit of snake identification since moving to a rural area 11 years ago, but luckily for me, with enough uncertainty to hedge my bets, and keep my distance, even when I ID them as safe.