Since its introduction at the boat shows this winter, a rubber life raft that completely inflates itself within 30 seconds has captured the imagination of U.S. yachtsmen. A commercial version of the self-inflating rafts of World War II, the 10-passenger model (there's a six-man raft, too) folds into a carrying case three feet long. It weighs 130 pounds when packed with equipment, can be inflated by a tug on a lanyard and afloat has a bright, easily sighted yellow canopy that doubles as protection against all weathers. U.S. Rubber, the manufacturer, is doling out distributorships carefully, insisting upon factory-trained technicians to repack the rafts and check them periodically. The sizes sell for $655 and $705 and offer the kind of disaster protection yachtsmen have sought for years.