Though a recent convert to the lure of fishing, Dorothea Lincoln Dean, here with moisture-beaded, champagne-filled trophy to commemorate her latest catch, has racked up in just one year of competition a reputation to be envied by old masters. In recent weeks she won the third annual sailfish tournament at Stuart, Fla. with two sails caught on 10-pound test line, then moved to the Ocean Reef Club billfish tournament at North Key Largo and won that with five sailfish, three of which were caught the last day on 12-pound test line. Mrs. Dean is also current holder of the women's record for blue marlin in the 50-pound-line class with a 384-pounder taken off Bimini. A native of Boston, where she properly meets her cultural responsibilities as an amateur pianist and composer (her Nocturne for Orchestra was played at a Boston Pops concert), Dorothea Dean is also an accomplished skier, mountain climber, duck hunter and skeet shot. And, like Hemingway's Old Man, she's running a private feud with sharks. They mutilated a big marlin that might have otherwise broken all previous records.