The small body of water that connects Nauset Marsh and the Atlantic Ocean and divides the beach with Nauset Beach on the south side and Coast Guard Beach on the north side is called Nauset Spit. It is not very wide, but wide enough for smaller boats to get through.
You can get to it by walking from Nauset Beach (a long walk) or Coast Guard Beach (a not as long walk). I took this photograph from the Coast Guard Beach side. Beautiful, don’t you think?