On July 30, 1999, approximately 2110 central daylight time, a Beech C23, N24670, piloted by an airline transport pilot, sustained substantial damage when it impacted water approximately 4 miles east of Port Washington, Wisconsin. The personal flight was being conducted under the provisions of 14 CFR Part 91 and was not on a flight plan. The pilot and his one passenger reported minor injuries. The flight had originated from the Griffing Sandusky Airport, Sandusky, Ohio, at 1800 and was en route to the Outagamie County Regional Airport, Appleton, Wisconsin. During a telephone interview, the pilot stated that, during the flight, he noticed the fuel quantity, as indicated by the fuel gauge, was decreasing even though the fuel selector was positioned for the left tank. The pilot stated that he decided not to change fuel settings since the engine was still running. The pilot stated that he was planning to divert to Sheboygen, Wisconsin rather than continue to Appleton. The pilot stated that while continuing to monitor the fuel status, he noticed that the right fuel tank quantity continued to decrease and the left fuel tank quantity had actually risen. The pilot stated that he continued to monitor the fuel status until the right fuel gauge read empty, at which time the engine quit. The pilot stated that he tried several times to restart the engine with differing fuel selector settings, but was unable to get the engine to run for more than a few seconds. The pilot stated that he had filled the fuel tanks prior to departure. Fuel receipts from the departure airport indicate that the aircraft was fueled with 19.6 gallons of fuel. As of December 27, 1999, the aircraft has not been recovered from Lake Michigan.