On February 13, 1997, about 1930 hours Pacific standard time, a Cessna 172F, N3757F, en route to Gnoss Field, Novato, California, lost engine power and ditched in San Pablo Bay, California. The aircraft sustained substantial damage. The pilot, the sole occupant, received minor injuries. The aircraft was operated as a personal flight by the pilot/owner when the accident occurred. The flight originated from the Schellville-Sonoma airport, Sonoma Valley, California, at 1920. Visual meteorological conditions were reported by the pilot and no flight plan was filed. The pilot reported that he had climbed to 1,000 feet msl when the engine began running rough. It began to lose power until the engine finally stopped and he was forced to ditch. After ditching, the aircraft partially sank in about 4 feet of water to the bottom of the bay. The pilot sprained his ankle during the water landing and decided to stay with the aircraft. He climbed on top of the aircraft and stayed there until morning. He was located by a Coast Guard Search and Rescue aircraft after he manually activated the ELT at first light. There was no flotation equipment onboard the aircraft. Rescue personnel reported the aircraft was found less than 0.25 mile from shore, at a position of 38 degrees 5.5 minutes north latitude by 122 degrees 24.7 minutes west longitude. The aircraft was recovered by Parker Diving Service after being in the water for several days. The pilot reported that the aircraft had last been refueled about 3 weeks before the accident, but had not flown until the day of the accident. His report indicates that he flew the aircraft a total of 30 minutes that day. At the time of the accident, the pilot estimated that the aircraft had about 28 gallons of fuel remaining. The aircraft had flown about 170 hours since the engine was overhauled by the owner/mechanic/pilot. The aircraft was equipped with a Brackett Air Filters air induction filter. An FAA Form 337, dated March 21, 1996, was on file for that nonstandard filter. An interview of the pilot and review of the aircraft logbooks by FAA airworthiness inspectors failed to identify any discrepancies. The 1945 weather observation at the Santa Rosa, California, airport, which is 36 miles northwest of the accident site, was reporting a temperature of 55 degrees and a dew point of 32 degrees Fahrenheit. A plot of these points on a carburetor icing probability chart discloses an area of the chart annotated "moderate icing - cruise power or serious icing - glide power." There was no postaccident inspection of the aircraft and the pilot/owner disposed of the wreckage after removing salvageable parts.