This is just a semi-educated guess, but I think you are looking at an old tidal canal. As the glaciers have melted over the years the land has risen and the tidal areas have receded. Now it is more like a freshwater pond, albeit a long, skinny one. If you follow the road out towards Eagle Beach, you can find this to your left, not far from the side. Wear rubber boots and don't get too close or you'll be swimming.