What is an adder? (I Iive in NH)?

My dad and I were walking our dog near a pond in Concord, NH. When our dog stepped in the water, we saw this big animal swim away leaving large ripples in the water. We umed it was a turtle or beaver, but when my dad asked some of his boating friends they said it was and adder (or atter i dont know how to spell it) When I searched for it I could only find the snake called an adder (which lives in Europe and Asia) but my dads friends insist that its a reptile with legs that can go on land and water. They even have said that they have seen them on docks. Could anyone tell me what this is or where to get a picture?