DEEP-WATER DIVERS have always been the daredevils of the diving community, pushing far into the dark labyrinths of water-filled holes and extreme ocean depths. It's a small global fraternity—there are no more than a dozen members—and in the history of recreational diving, only six people other than Shaw have ever pulled off successful dives below 820 feet. (More people have walked on the moon, Don Shirley likes to point out.) At least three ran into serious trouble in the process (including Nuno Gomes, who got stuck in the mud on the bottom of Bushman's Hole for two minutes before escaping)....