The pilot reported that he departed with 74 gallons of fuel onboard for the flight of about 84 nm. After an uneventful flight the pilot selected the runway that was favored by prevailing winds. He then extended the landing gear and reduced power while the airplane was on the downwind leg of the airport traffic pattern over the ocean. The pilot encountered a partial loss of engine power as he turned to the base leg and the airplane began to descend from 800 ft above ground level. He performed emergency procedures for a loss of power from memory, but the engine continued to run at low power. Unable to maintain altitude, the airplane impacted the ocean and sank. The pilot and passenger, who were both seriously injured, egressed the airplane and swam to shore. Subsequent efforts to locate and recover the airplane were unsuccessful.