In March 1999, Shelley Tyre, the principal at the prestigious Thayer Academy just outside Boston, and her husband, David Swain, a single father and owner of a dive shop in Jamestown, Rhode Island, took a vacation to the Island of Tortola in the Caribbean. The two had met through their shared passion for adventure and this was their dream vacation - sun, sailing and scuba diving. But it would turn out to be Shelley's last. On the final dive of their trip, Shelley and David went into the water together, but only David returned to the surface. Something had gone mysteriously wrong that day and now Shelley was dead. David Swain couldn't explain what happened to Shelley during the fatal dive. He admitted the two had split up underwater, forgoing the standard safety rule of using the buddy system. Did she suffer a heart attack or a stroke? Or was it panic? While Tortola authorities ruled Shelley's death an accident, her parents had their doubts, especially after David began showing a bizarre lack of remorse in the wake of her death. David's two children adamantly defend their father and say that he and they loved Shelley and that he could never have killed her. Was Shelley merely the victim of a mysterious accident and David the victim of a past trauma that left him emotionally unable to appropriately cope with her death? Or, did David's violent past ultimately lead him to commit murder? David Swain and his children speak for the first time since he was charged with murder in Troy Roberts' report on "48 Hours Mystery: "Shelley's Last Breath".