Astronautilus

Hi folks... I'm looking for a title that I read a few years ago... it's an adventure yarn, set in modern times. The hero is a rich playboy with a sailing yacht (all mahogany and brass), who goes to a small island somewhere off the New England coast in search of old (pirate?) treasure. The plot involves digging an immense shaft in the centre of the island. Of course there are other factions looking for the treasure too... Any ideas anyone?