On April 17, 2021, about 1715 eastern daylight time, a Cessna 172 airplane, N60457, was substantially damaged when it was involved in an accident near Venice Municipal Airport (VNC), Venice, Florida. The flight instructor and private pilot receiving instruction were not injured. The airplane was operated by Florida Flight Training Center as an instructional flight conducted under the provisions of Title 14 Code of Federal Regulations Part 91. According to the flight instructor, the private pilot receiving instruction’s last flight review was in 2008. The accident flight was the second flight working towards a flight review signoff. They departed with full fuel and completed maneuvers in the local area before the return to VNC for touch and go landings. The flight instructor reported that he demonstrated the first touch-and-go, which was uneventful, and then the student took the flight controls for the second touch-and-go. While in the traffic pattern on downwind, abeam the runway numbers, the student pilot intended to reduce engine power to 1,700 rpm, however, as he did the rpm dropped to 1,000. The instructor observed the student attempt to increase the power, but the there was no change in the rpm. The flight instructor reported that he immediately took the flight controls and attempted to troubleshoot the engine issue, but the engine only produced two “weak vibrations” and he was unable to restart it. He added that it became apparent that their glide path would not allow them to make the runway, so he turned right to land on a golf course fairway and pond. Subsequently, the left-wing contacted trees and the airplane came to rest partially submerged in a pond where both pilots safely egressed. The fuselage, wings, and empennage sustained substantial damage. The engine remained submerged in water for about 18 hours before it was recovered. During the recovery the wings were removed and about 35 gallons of fuel was recovered. A sample of that fuel revealed it was blue and absent of debris or water. On May 10, 2021, the engine was examined by a National Transportation Safety Board investigator. Visual examination of the engine revealed that all major accessories, fuel lines, and control cables were intact and secure. Additional fuel samples were taken from the belly sumps, and firewall fuel filter; the fuel was found to be blue and absent of debris and water. When the bottom spark plugs were removed, dark water was observed draining from 3 of the 4 cylinders. The spark plugs were dark in color and three of the plugs exhibited some corrosion and debris from being exposed to water. The valve covers and the ignition leads for both magnetos were removed, and the propeller was manually rotated. Valvetrain continuity was established on each cylinder and the impulse couplings for both magnetos could be heard clicking. However, spark was only observed to two of the left magneto’s ignition leads as the propeller was rotated. The top plugs were then removed, and they too appeared dark and exhibited corrosion and debris. The spark plugs were cleaned, and a small gas tank was plumbed into the right-wing fuel port. Several attempts to start the engine were unsuccessful. The magneto caps were removed to the left and right magnetos, and corrosion was observed in the cap and interior housing of both magnetos. An attempt was made to dry the right magneto and remove any visible corrosion; however, the engine would not start. A brand-new right magneto was placed on the engine and timed. The engine subsequently started immediately and ran without interruption through its full power range. Review of maintenance records found that the most recent 100-hour inspection was completed on March 25, 2021. The inspection documented routine maintenance actions and the airplane had flown about 40 hours since that inspection. An attempt to retrieve data from the primary and multi-function display systems was made, however, the SD cards contained no data.