On November 10, 2019, about 0920 eastern standard time, a Mooney M20J, N5765H, was substantially damaged during a forced landing in a marsh, while on approach to Northeast Florida Regional Airport (SGJ), St. Augustine, Florida. The airline transport pilot sustained minor injuries and the passenger was seriously injured. The personal flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and no flight plan was filed for the flight that departed Gainesville Regional Airport (GNV), Gainesville, Florida, about 0820. The pilot reported that he was repositioning the airplane from the facility that performed the annual inspection, back to his home airport. While on an extended base leg of the airport traffic pattern for runway 31, he reduced airspeed from 100 to 95 knots and then increased engine power to maintain 95 knots. At that point, the engine began to surge for several seconds, followed by a total loss of engine power. The pilot verified that the fuel boost pump was on, the mixture was rich and the fuel selector was positioned to a correct fuel tank. The pilot also checked both magnetos with no success. He then retracted the landing gear for better glide performance but realized that the airplane would not glide all the way to the runway. The pilot subsequently performed a forced landing to a marsh about 1 mile prior to the runway threshold. Initial examination of the airplane by a Federal Aviation Administration (FAA) inspector revealed that it came to rest upright, mostly submerged in shallow water. The inspector noted damage to the right wing and empennage. Once the wreckage was recovered to shore, the inspector observed the fuel selector positioned to the right main fuel tank and about 20 gallons of 100-low-lead aviation gasoline was drained from the right main fuel tank. Water was drained from the left main fuel tank. The water had a smell of fuel, but the inspector did not observe any 100-low-lead aviation gasoline mixed with the water. The airplane was examined again by an FAA inspector at a recovery facility. A visual inspection of the engine did not reveal any obvious defects and it exhibited corrosion due to submersion in brackish water. The fuel injector divider was opened and no fuel was found; however, a white crystalized power similar to salt was observed, consistent with the submersion in brackish water. Each individual fuel injector was removed and inspected. All the injectors exhibited evidence of the white crystalized powder and no blockages were found. The top spark plugs and ignition leads were removed and no anomalies were observed. The air filter and air box were also examined and no anomalies were observed. The engine was subsequently examined by a National Transportation Safety Board investigator. Visual inspection of the engine and propeller did not reveal any preimpact mechanical malfunctions. The rocker arm covers were removed and no anomalies were noted. The spark plugs were removed and appeared gray in color. A lighted borescope was used to inspect the cylinders and valves, with no anomalies noted. The propeller was rotated by hand and thumb compression was established on all cylinders. The magnetos and fuel pump were corroded and could not be tested. The four-seat, low-wing, retractable tricycle landing gear airplane was manufactured in 1984. It was powered by a Lycoming IO-360, 200-hp engine equipped with a three-blade, constant-speed, Hartzell propeller. Review of maintenance records revealed that the airplane's most recent annual inspection was completed on October 30, 2019. At that time, the airframe had accumulated 3,398.4 total hours of operation and engine had accumulated 1,610.8 hours since major overhaul. The airplane had flown about 1 hour from the time of the most recent. annual inspection, until the accident, which was the accident flight.