The Northern Water Snake is harmless. There are ZERO poisonous
aquatic snakes in New Hampshire.
I see them in Florida, while I'm working in knee-deep water. They don't pay any attention to me, as they leisurely swim past my knees.
Their patterns are similar, so they can't be readily distinguished from the aggressive (and poisonous) Water Moccasin, except the Northern Water Snake swims completely under water. (The Moccasin swims with head exposed).
Northern Water Snakes can make themselves look angry--and some have grown to four feet long!