AMATHUS, Cyprus—It’s said that Demetrius the Besieger, a mighty warrior king and one of Alexander the Great’s successors, built this harbor on Cyprus’s southern coast 2,400 years ago to thwart a potential naval invasion from the ruler of Egypt, Ptolemy I, another of Alexander’s heirs. French archaeologists who initially studied the ancient harbor of Amathus believe it to be an incomplete military fortification work, the three piers of which would have accommodated the best of the ancient world’s naval ships, ready to repel an attacking force. Lying just a few feet underwater a mere 200 feet off the coastline near the resort town of Limassol, the harbor will soon be Cyprus’s newest tourist attraction where adventurous holidaymakers can snorkel over its submerged stone remains. It’s a novel direction for Cyprus’s tourism authorities, who are looking beyond the east Mediterranean island nation’s long-held “sun and surf” product to reach out to …