That sounds like a Water Moccasin. Nasty pit vipers which can be very aggressive. I've seen many in Eastern PA, not sure how far north they go, but I'm sure around here in the gunks seeing one would not be out of the question.
Coloration can vary, Most are dark grey/brown, some are more brown, and most have a barely noticeable pattern. They are not shiny. All will swim with their heads sticking above water and body just at the water surface. They are usually found lying out getting sun on rocks near water or wet lands. Ive only had one come after me, most will retreat to the water as you have witnessed, but if threatened they will stand their ground.