On July 6, 2002, at 1339 eastern daylight time, a Beech C23, N823DA, was substantially damaged during a forced landing, after departure from Plum Island Airport (2B2), Newburyport, Massachusetts. The certificated commercial pilot and the private pilot were not injured. Visual meteorological conditions prevailed for the flight destined for Beverly Municipal Airport (BVY), Beverly, Massachusetts. No flight plan had been filed for the business flight that was conducted under 14 CFR Part 91. According to the commercial pilot, the purpose of the flight was to take a company employee from Beverly, Massachusetts (BVY), to Plum Island, and return the airplane to Beverly. A private pilot occupied the left seat and was the pilot flying, while the commercial pilot occupied the right seat. The flight to Plum Island was completed without incident. The passenger disembarked without the engine being shut down, and then the private pilot taxied the airplane to runway 28 for departure. After takeoff, he performed a left downwind departure, and flew the airplane east toward the island's shoreline. As the airplane neared the shore line, at an altitude of about 1,000 feet, the private pilot reduced engine power to a cruise setting, and noticed that the engine power continued to further reduce to near idle. The commercial pilot immediately took control of the airplane and performed a 90 degree turn to the left, to position the airplane on a left base back to runway 28. The commercial pilot observed that he was too far from the airport to glide to it, and then made a 180 degree turn to the right to follow the shore line to the south. The commercial pilot reported that the beaches were crowded with people. While descending, the commercial pilot changed the fuel selector to the other wing tank, checked carburetor heat, and the magneto switch. His attempts to keep the engine running were not successful. At one time, the engine momentarily surged, but then lost power again, and he was still unable to restore power. The commercial pilot also reported that he initially thought about landing on a straight stretch of road that ran north and south. As he continued toward that stretch of road, he observed one vehicle, which did not clear the road. At an altitude of about 50 feet AGL, the commercial pilot side-stepped to the right, to land in the marsh. The nosewheel struck an elevated portion of the terrain and collapsed. The nose then dug in, and the airplane came to rest inverted. According to a Federal Aviation Administration (FAA) inspector, the engine had been pushed rearward, and the firewall was wrinkled. Some of the engine mounts had been fractured. Thumb compression was attained in all cylinders. Fuel flow continuity from the fuel tanks to the carburetor was confirmed. The carburetor fuel screen was absent of debris, and the fuel appeared clean. The spark plugs were not fouled. Electrical continuity was confirmed to the magneto switch. The baffles in the exhaust muffler were in place. The engine was test run satisfactorily on the airframe, utilizing the airframe fuel system. The engine was not run above idle due to broken engine mounts and vibration. The carburetor was examined under the supervision of an inspector from the FAA. According to the report, the carburetor flowed 17 percent richer than normal at idle. At the mid-range power settings, the richness decreased to about 5 percent, and at higher power settings, the richness was about 2 1/2 percent.