On May 18, 2023, about 1817 Pacific daylight time, a Cessna 172H airplane, N2550L was substantially damaged when it was involved in an accident near Seattle, Washington. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that, while in cruise flight at 1,000 ft msl nearing the destination airport, he heard an audible change in engine rpm. The engine tachometer indicated 1,400 rpm instead of the expected 2,000 rpm. There were no other abnormal engine sounds or indications on the instrument panel. The pilot attempted to troubleshoot; however, the engine rpm continued to decrease. No longer able to maintain altitude, the pilot declared an emergency and ditched the airplane into a bay, avoiding populated areas. A postaccident examination of the engine revealed the No. 4 cylinder exhaust valve was stuck in the partially open position. The intake valve was removed with little effort; however, the exhaust valve was removed with a punch and hammer along with significant force. The exhaust valve guide exhibited evidence of oil coking along with an area of polishing on one side of the guide. A slight amount of discoloration consistent with corrosion was observed adjacent to the polished area. The exhaust valve stem was intact with some coking near the valve fillet. The intake valve had moderate coking. The cooling fins on cylinder Nos. 2 and 4 exhibited thermal discoloration. All intake and exhaust valve springs were found intact and undamaged. The intake and exhaust pushrods were intact and undamaged. The No. 4 piston exhibited evidence of corrosion and combustion deposits were observed on the piston face. All piston rings were intact and had various degrees of corrosion and contamination throughout. The internal area of the crankcase, the camshaft, and crankshaft were found to be unremarkable. The aircraft maintenance logbook revealed that the engine had a major overhaul September 8, 1995. On March 16, 2022, the No. 4 cylinder was serviced and reinstalled with a new cylinder kit. The exhaust and intake valve guides were reamed, and the valves were reinstalled. At that time, the engine had accumulated 1,021.8 hours since major overhaul. The airplane had 143.2 total flight hours from the time of the No. 4 cylinder servicing until the accident and the engine had 1,165 total hours since new at the time of the accident. The airplane’s most recent annual inspection was completed on June 8, 2022. The annual inspection included replacement of the No. 4 cylinder exhaust stack. The engine manufacturer recommended overhaul time for the engine was 1,800 hours or 12 years, whichever occurred first.