The airplane crashed into a swamp at about 1211 Central time; visual conditions prevailed and an IFR flight plan was in effect for the flight from Houston, Texas, to Panama City, Fla. The airplane was substantially damaged and the Private pilot was fatally injured; a passenger sustained minor injuries. While en route, the passenger was awakened by his father, the pilot, and advised to put on his seatbelt and shoulder harness. His father also advised him that the engine was running rough. The passenger reported that his father was talking with someone on the radio, and the aircraft was then in the trees.