On July 23, 2012, at 1502 eastern daylight time, a Piper PA-25, N7313Z, registered to and operated by Simmons Aviation, was substantially damaged following a forced landing into the Atlantic Ocean, near Westerly, Rhode Island. The certificated airline transport certificated pilot was not injured. Visual meteorological conditions prevailed at the time and no flight plan was filed for the local flight, which originated from Westerly State Airport (WST), Westerly, Rhode Island, about 1445. The banner tow flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 91. The pilot stated that he departed with 25 gallons of fuel for a 45-minute flight, and had not taken on any fuel that day. About 12 minutes into the flight, and while flying at an altitude of about 600 feet, the engine began running "roughly" and sounded as though it misfired. After about 20 seconds, the pilot responded by increasing engine power to full, richening the mixture, and applying carburetor heat. Shortly after, the engine experienced a total loss of power. The pilot subsequently ditched the airplane into the ocean with the banner still attached, approximately 6 miles from shore. The pilot transmitted a distress call, and was later rescued by a civilian boat. The airplane was recovered from the ocean on July 31, 2012. The pilot performed a cursory examination of the engine and noted that the crankshaft was free to rotate, and oil was present within the engine case. The pilot reported that there was an unspecified “problem” with a valve of the number 4 cylinder, and that he was unable to check operation of the magnetos since they were saturated with water. An unspecified quantity of fuel was found in a fuel tank. Due to the extent of damage related to immersion in salt water, no further examination of the engine was possible. The pilot had most recently purchased fuel from a fixed base operator at WST. The fuel facility there was inspected at the direction of a Federal Aviation Administration (FAA) inspector following the accident. No abnormalities were noted during the inspection. The accident airplane was manufactured in 1965 and was equipped with a Lycoming O-540-B4B5 engine. Review of maintenance records showed that the airplane’s most recent annual inspection was completed on August 7, 2011. At that time, the airframe had accumulated 4,720 total flight hours, and the engine had accumulated 2,147 hours since the most recent overhaul. The airplane had accumulated an additional 150 flight hours since that time. The pilot held an airline transport pilot certificate with numerous ratings, including airplane single engine land. His most recent FAA second class medical certificate was issued in June 2012, and he reported that he possessed 9,175 total hours of flight experience, 1,500 hours of which were in the accident airplane make and model. The weather reported at WST, at 1500, included winds from 210 degrees magnetic at 9 knots, 10 statute miles visibility, a broken ceiling at 1,600 feet, temperature 26 degrees Celsius (C), dew point 21 degrees C, and an altimeter setting of 29.98 inches of mercury. The temperature and dew point conditions at that time were conducive to the formation of serious carburetor icing at glide power settings.