On July 16, 2019, at about 1815 eastern daylight time, a Cessna 172RG airplane, N9715B, sustained substantial damage during a forced landing following a partial loss of engine power near Ocean City, Maryland. The solo commercial pilot was uninjured. The airplane was registered to and operated by the pilot, under the provisions of 14 Code of Federal Regulations Part 91 as an aerial survey flight when the accident occurred. Visual meteorological conditions prevailed and no flight plan had been filed. The flight departed Reedville Airport (VA98), Reedville, Virginia. According to the pilot, at 3,000 ft about 8 miles offshore while conducting fish spotting operations, the engine began to shake and lose rpm. In an effort to avoid ditching in the ocean waters, he navigated to the shoreline and the nearest airport. When it became evident he was unable to reach the airport, he ditched the airplane in the shallow waters of the ocean surf to avoid bystanders on the beach. The airplane sustained substantial damage to right elevator. Examination of the Lycoming O-360 series engine revealed that the engine's No. 2 cylinder exhaust valve spring and guide had liberated and were located inside the damaged rocker cover. The piston and interior of the No. 2 cylinder exhibited mechanical impact damage and a hole was present in the face of the piston. The exhaust valve and exhaust valve keepers were not located. Maintenance records revealed that the airplane's most recent annual inspection was completed on April 2, 2019. The most recent engine maintenance record entry dated June 17, 2019, indicated that the No. 2 cylinder had been removed and replaced with an overhauled cylinder. According to a mechanic who maintained the airplane, the aircraft and about 10 others flew about 1,000 hours a year for fish spotting operations. He said that stuck valves were common, as the airplanes are operated at low altitudes with rich engine fuel/air mixtures. He went on to state that the No. 2 cylinder on the accident airplane had been replaced due to a stuck valve that contacted the piston, which necessitated replacement of the entire cylinder assembly. The closest weather reporting facility is Ocean City Municipal Airport (KOXB), Ocean City, Maryland. At 1753, a METAR from KOXB was reporting in part: wind, 190° at 8 knots; visibility, 10 statute miles; sky condition, clear; temperature, 84° F; dew point 75° F; and an altimeter setting of 30.05 inches of mercury.