(FORTUNE Magazine) – Once the exclusive domain of macho-suicidal types who watched Sea Hunt on television and dreamed of wrestling with giant octopuses, scuba diving is . growing fast as a national recreation. About 400,000 Americans, one-third of them women, became certified divers last year, bringing the total of active plungers in the U.S. to nearly three million. Sales in the U.S. of fins, air tanks, and other scuba gear made by such manufacturers as U.S. Divers, Dacor, and Scubapro have been growing at 8% a year since 1985, reaching $500 million in 1989. Exports are rising at 20%. Diving in such exotic destinations as Belize, Bonaire, and the Cayman Islands is also booming. Closer to home, the waters of John Pennekamp Coral Reef in the Florida Keys have become a mecca for vacationing executives wearing wet suits. Says park manager George Jones: ''The demographics of the visitors have changed dramatically in the past five years.'' -- M.A.