On November 5, 2011, at 1045 eastern daylight time, a Siai-Marchetti S205-22R, N729WA, experienced a partial loss of engine power during climbout from the River Acres Airport (FD70), Okeechobee, Florida. The pilot performed a forced landing in the Kissimmee River, which resulted in substantial damage to the fuselage. The private pilot was not injured. The airplane was registered to and operated by a private individual under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed and no flight plan had been filed for the flight destined for North Palm Beach County General Aviation Airport (F45), West Palm Beach, Florida. The flight was originating at the time of the accident. The pilot stated that during the climbout, approximately 200 feet above ground level, the engine lost partial power and was running rough. The pilot verified that the mixture was in the full rich position and applied carburetor heat in an unsuccessful attempt to restore the engine to full power. He maintained airspeed and elected to land in the Kissimmee River which was straight ahead. The fuselage buckled during the forced landing. Earlier in the day, the pilot completed a preflight inspection and noted that the engine contained 8 quarts of oil. In addition, the pilot fueled the airplane with about 30 gallons of fuel prior to flying to FD70. An FAA inspector completed a post accident examination of the engine and noted that it was partially seized and oil was present. In addition, the alternator drive adaptor did not function. The alternator drive adaptor was removed and disassembled. The alternator drive shaft was bent and the drive gear exhibited extensive wear and the metal teeth were smeared. The alternator drive bearings were not present and metal particles were found throughout the oil system in the engine. In addition, the engine alternator drive belt that was driven by the engine driven alternator drive adaptor was not located. The alternator was on the maximum setting of the alternator adjustment link. Review of the maintenance records revealed that on October 21, 2011, the alternator was replaced by the pilot/owner and was signed off by an airframe and powerplant mechanic. At that time, the airplane had accumulated 2,116 total hours of flight time. At the time of the accident, the airplane had accumulated 2,118 total hours of flight time.