We have the diamond back water snakes in our ponds but they are more the brownish color like the water moccasin pictured. But can definitely tell by the head that ours is not poisonous. The poisonous snakes have that broad more diamond shaped head where the poison glands are. Non-poisonous heads more slender. At least that is the way it appears to me.
IIRC I read somewhere you can also tell by the pupils in their eyes. Nonpoisonous have round pupils and poisonous are verticel elliptical shape.
I dun plan to get close enough to get that good a look!
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