Gallows on the Sand — Morris West

Twenty chests of minted Spanish gold in a sunken galleon-this is the lure that brings historian Renn Lundigan to a tiny island off Australia's Great Barrier Reef. Renn enlists islander Johnny Akimoto to teach him scuba diving, but soon realizes there's a far greater danger in the reef than the sharks. Gambling den owner Manny Mannix has followed him to the island, and now he threatens not only Renn and Johnny, but also the beautiful young scientist Pat Mitchell. Gallows on the Sand is a fast-paced story of high adventure, with the rich characterization that made Morris West one of the bestselling writers of his day.