It's a Northern Water Snake--a really big, beautiful one, too.
Cottonmouths don't LIVE in Massachusetts.
There are only two venomous snakes in Massachusetts--the Timber Rattlesnake, and the Copperhead.
Water snakes are notoriously bad-tempered.
Edit: My bad, you were in Maine. Maine doesn't have copperheads, or timber rattlers. Or any other venomous snake species, for that matter, so feel free to go about your grabbing or netting with impunity. lol