yep...water snakes of various species have triangular heads....not AS triangular as the pit vipers, but enough that the ignorant will kill them and claim cottonmouth.
That head looks like any and every cottonmouth head I've ever seen, or seen pictures of. Did you actually see the body, or was just the head left. A herpetologist from college taught us that the barring on the eye was the sure fire way to tell a cottonmouth, especially when they got big and dark/or muddy (although the body of a cottonmouth is almost unmistakeable as a large adult, most being FAT FAT snakes).