The float-equipped airplane was on a local area flight when the pilot noted a partial loss of engine power. The pilot's corrective actions did not restore full engine power and the airplane was unable to maintain altitude. He reduced engine power to flight idle and entered a rapid descent into a nearby lake. He noted that the engine continued to operate while in the flight idle position. He reported that due to the size constraints of the lake, the landing was "necessarily firm" and that the airplane nosed over shortly after touchdown. The fuselage primary structure, left horizontal stabilizer, right float and support structure, and left wing spar were substantially damaged during the accident. A post-accident inspection confirmed flight control continuity existed between the cockpit controls and the individual flight control surfaces. Fuel system continuity was established between the fuel control and carburetor. No mechanical malfunctions or failures were identified with the engine during the post-accident inspection.