Divers exploring the murky waters of a former shipyard at Benicia, 30 miles northeast of San Francisco, said they think they have found the sunken hull of a Boston whaler that disappeared from the record books in 1893 after 50 years of sailing. "We are convinced it is the Stamboul," said Peggy Martin, research historian with the Benicia Historical Society. Built in 1843, the Stamboul was a 260-ton whaler measuring just over 106 feet, and divers said it rests 120 yards off the shoreline. "What you have here," said Jim Delgado, a U.S. Park Service archeologist, "is a tangible link to a time when the United States was primarily a maritime nation . . . a vessel that comes right out of the pages of Moby Dick."