Some watersnakes are somewhat hard to identify by looks alone, without locality data. But it does look to be a Northern Watersnake(Nerodia sipedon),..... or possibly a form of
"erythrogaster"
In any case,....it's just a harmless watersnake, and nothing venomous!. They typically flatten their heads in defense when feeling threatened.
Also,.....the Brown Watersnakes(Nerodia taxispilota) are much more distinct looking, having a tannish brown coloration with VERY distinct square shaped blotches/saddles on the back.
best regards, ~Doug
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"Better to be silent and thought a fool, than to open mouth and remove any doubt!"