Trouble in Paradise — Franklin W. Dixon

A mission for ATAC has Frank and Joe Hardy traveling the world.

The teenage son of a prominent mid-eastern diplomat has gone missing on the Caribbean island of St. John, and the boys take off to find him. Turns out he's been kidnapped by a group involved in a scheme to rob a sunken Spanish galleon of its treasures and sell them on the black market. Frank and Joe search high and low--fighting thugs and sharks--for clues. When the Hardys finally find their quarry, there's another problem --he turns out to be the mastermind of the ring, not its victim!