I'm not an expert on this, but my understanding is that along oceans, the public only has unfettered access to the area of property exposed below the mean high water line, i.e., when the tide is out (federal law). And I believe that along inland lakes and rivers, the property owner owns to the water line, period. If the water level goes down they gain land, and if the water level rises, they lose land. I hope somebody more expert will correct me on this if I am wrong.

And I won't get into the issue of riparian rights, because that is way too complicated.