I'm guessing it was probably one of these....
https://en.wikipedia.org/wiki/Northern_water_snake
Our new house has a cluster of them that have lived nearby for years. My children have named the largest one "Bob"
It was pretty unnerving seeing them around for the first year or so. Especially one time when I looked down to see one swimming under me.
That said I spoke to the naturalists at the Squam Lake Science Center and they told me the snakes are pretty harmless but its important to leave the alone. They are skittish and will generally run away from you but if try and corner one they can get pretty fierce. The bite isn't poisonous but it hurts like heck I'm told.
Oh and they are definitely to blame for the frogs disappearing.