The aircraft experienced a loss of engine throttle control at about 0955 Eastern time, resulting in a forced landing short of Runway 31 at the St. Augustine Airport. Visual conditions prevailed for the flight from Green Cove Springs, Fla., to St. Augustine. The airplane was substantially damaged; the Private pilot, the sole occupant, was not injured. On final approach, the pilot reported the throttle …was just loose in my hand (like it completely disconnected from the carburetor). Unable to increase power, he ditched the aircraft in a marsh short of the runway. The airplane flipped inverted during touchdown. The engine throttle control was found separated from the throttle control arm of the carburetor.