On September 19, 2003, about 1155 central daylight time, a Cessna 172K, N79491, registered to and operated by Omega Protein Corporation, was substantially damaged during a forced ditching in the Gulf of Mexico near Ocean Springs, Mississippi. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 CFR Part 91 aerial observation (fish spotting) flight. The commercial-rated pilot, the sole occupant, was not injured. The flight originated about 0905, from Trent Lott International Airport, Pascagoula, Mississippi. The pilot stated that he flew the accident airplane 2 days before the accident date for approximately 4.0 hours and after landing, the airplane was serviced. He also flew the accident airplane the day before the accident date a total of 5.5 hours; after landing a total of 37.0 gallons of fuel were added to the fuel tanks. On the morning of the accident, he arrived at the departure airport about 0800, and departed about 0905. Approximately 1 hour 25 minutes after takeoff (about 1030), he began transferring fuel from the auxiliary fuel tank, and the flight continued. At about 1140, he proceeded to return to the departure airport for shift change, and approximately 15 minutes later, he experienced, "engine trouble" which consisted of, "spitting and sputtering." He switched the fuel selector back and forth numerous times with no response, along with switching the electric fuel transfer switch on and off, with no effect. He reported the fuel transfer switch had been on since 1030 hours. He initiated emergency procedures, ditched the airplane which went inverted, and exited the airplane. The pilot reported to FAA personnel that he believed there was a malfunction with the auxiliary fuel tank pump which transfers fuel to the right fuel tank. An FAA airworthiness inspector examined the airplane the afternoon of the accident while it was in the water, and again the next day following recovery of the airplane. Examination of the airplane the day after the accident revealed both fuel tanks contained approximately 1/4 quart of fuel; the fuel drained from the left tank was reported to be cloudy, while approximately 1/2 quart of water was also drained from the right fuel tank. The gascolator was noted to have a thimbleful of clean fuel. The installed 18-gallon auxiliary fuel tank was noted to be nearly full of fuel. Operational testing of the auxiliary fuel tank pump was successful; the pump transferred approximately 15 gallons of fuel into the right fuel tank in about a 3-hour period. The 18-gallon auxiliary fuel tank was installed in the airplane on December 15, 1989, in accordance with Supplemental Type Certificate SA 615NE. According to the holder of the STC, the auxiliary fuel tank transfer pump is designed to transfer 8 gallons-per-hour from the auxiliary fuel tank into the right main fuel tank.