(case involves public right to use beach located along side of an island; trial court determined that defendant had demonstrated existence of prescriptive easement on public’s behalf and ordered fence to be removed that plaintiff had erected; trial court decision based on presumption of adverse use and that plaintiff had to prove permissive use; reversed on appeal because beach at issue was unimproved and in general state of nature such that presumption should have been that public use was by permission of owner).