Harmless watersnakes (Nerodia sp.) are oden mistaken for venomous cottonmouths (Agkistrodon piscivorous/conanti) in the eastern US, even outside of cottonmouth range (the southeastern Atlantic coastal plain). Here is a nice graphic depicting visual differences in their body shape, face, and pattern. Note: this doesn’t include all North American watersnakes, but it includes the species more likely to be encountered in the same range.