A scuba diver unearths a sunken U-boat that holds a terrible secret   Robert Moore had a cushy life in Baltimore. The son of a bank president, he could have had the old man's job if he'd just waited in line. But Moore isn't the patient type, and rather than spend his life trapped behind a desk, he decamped for the Caribbean, to pass his days diving beneath the perfect blue sea. One day, diving deeper than usual, he spies a sunken ship. His investigations disrupt an unexploded depth charge, which hurls Robert to the surface with the sunken ship not far behind.   The U-boat, still seaworthy after all these decades, drifts towards the island and gets caught on the reef. A strange knocking echoes from inside the hull, as though something within is still alive. When Robert opens the long-closed hatch, he'll learn that some sunken treasure is better left undisturbed.