After retrieving the notes, the boys stick around to help Perry raise a yacht belonging to a wealthy businessman. In the process, they become entangled in Perry's bitter feud with another diver, Gus Kuntz, the unscrupulous owner of a rival diving company. Kuntz employs the thugs Bock and Simon, and repeatedly tries to sabotage Perry's diving operations, putting Perry's and the boys' lives at risk. Mysterious notes left at the Hardys' hotel serve as the eponymous secret warnings, foretelling misfortune, but the boys are undaunted.