Reef of Death (The Zone Unknown) By Paul Zindel

P.C. McPhee is in Australia to help his uncle solve an underwater mystery involving buried treasure. P.C. quickly figures out that they're not the only ones looking for treasure -- an evil geologist is determined to get her hands on it at any cost. P.C., his uncle, and the Aboriginal girl, Maruul; become ensnared in a deadly game of cat and mouse where the winner takes all, including the lives of the loser