On January 14, 1997, about 1240 eastern standard time, a Waco UPF-7, N29986, registered to Barnstormer Air Inc., operating as a 14 CFR Part 91 sightseeing flight, experienced a reported loss of engine rpm during cruise flight. The pilot made a forced landing to the beach in the vicinity of Sanibel, Florida. Visual meteorological conditions prevailed and no flight plan was filed. The commercial pilot and two passengers reported no injuries. The airplane sustained substantial damage. The flight originated from Fort Myers, Florida, about 25 minutes before the accident. The pilot stated that he was in cruise flight at about 1,000 feet, when the engine began to run rough. He turned the carburetor heat on and switched the fuel tanks with negative results. He made a forced landing downwind due to personnel on the beach. On landing rollout the airplane swerved to the left, nosed over into the water, and came to rest inverted. Examination of the engine assembly and accessories was conducted by the FAA. The results of the examination were inconclusive .