This is a garter. A bigger one at that. I doubt it was a garter if it was close to 5 feet long. If it was indeed 5' it would be an impressive specimen.
I have noticed this as well, if it's swimming it must be a moccassin, on dry land it's either a copperhead or a rattler. People don't even know that almost every snake can swim just fine, and I have never seen a long shiny black copperhead. I have a tendency to stop and look, so I'll actually know what I saw, mostly black racers....But of course for all my family and friends they are ALL moccasins and copperheads.