Caribbean Pirates Puzzle

Somewhere in the Caribbean sea at 2nd July 1816... In a dark and stormy night Captain Sparrow the most overrated pirate in the Caribbean and his mob of bloodthirsty pirates pounded the frigate Meduse. Finally after a long and hard fight, they boarded the Ship. Just very few of the crew survived the strike. But on board there were no wealth at all. Just a little treasure-map was found in one of the passengers possession. Unfortunately he was overhasty killed by the big fat and stupid pirate Blood-Svente before somebody could find out any details about the map. Ultimately the frigate with the rest of the surviving crew was deadlocked at the Arguin-Sandbank and set on fire. Captain Sparrow's ship the Black Earl and the mob of pirates sailed the Caribbean sea and followed the maps coordinates. The island on the map looked very strange. It was nearly a round island in such a perfection...