If you are a herpetologist, can you please help me identify these snakes living in my sister's pond?

We have some photos which I can PM you. They resemble northern brown or water snakes but do not seem to have the mottling. They are several feet long and as thick around as a woman’s wrist. This is an area with cold winters in the central part of New York state. We have small children who want to swim in the pond, but we need to ID the snakes first. Apparently not poisonous, they do seem agressive.