On October 21, 2006, about 1400 Pacific daylight time, a Helio H-295 floatplane, N6471V, experienced a partial loss of engine power on takeoff, landed hard during the ensuing precautionary landing and sank in Lake Isabel, Washington. The private pilot received minor injuries and his passenger received serious injuries. The airplane is presumed to have sustained substantial damage. The airplane was registered to Berger Leasing LLC of Monroe, Washington. Visual meteorological conditions prevailed and no flight plan was filed for the 14 CFR Part 91 personal flight. The airplane had departed from Firstair Field Airport in Monroe about 1340, landed on Lake Isabel, and was departing for the return flight to Monroe when the accident occurred. According to the pilot, he landed the airplane on the lake and back taxied for takeoff. The takeoff was normal until the airplane reached an altitude of 50 to 75 feet at which point he noticed that the engine was producing only "2/3rds power with full throttle." The pilot was concerned about not being able to clear trees at the end of the lake if he continued straight ahead, so he elected to execute a 180 degree turn and land back on the lake. As the steep left turn was completed, the airplane lost lift, and the landing was hard. The airplane began to rapidly fill with water, and the pilot and passenger exited through the rear cabin door. As the pilot and passenger were swimming to shore, the airplane completely submerged. According to a representative of the owner, as of December 6, 2006, there are no plans to recover the airplane from the lake. The reason for the partial loss of engine power could not be determined. The owner's representative stated that the passenger sustained "cracked ribs."