On April 7, 1998, about 1930 eastern daylight time, a Brock 162F, experimental helicopter, N623LB, registered to a private owner, operating as a 14 CFR Part 91 personal flight, experienced a decay in rotor rpm on initial climbout from a private unregistered heliport. A forced landing (autorotation) was conducted into Lake Altoona, located in the vicinity of Canton, Georgia. Visual meteorological conditions prevailed and no flight plan was filed. The helicopter sustained substantial damage. The private pilot reported minor injuries, and the passenger reported serious injuries. The flight originated about 2 minutes before the accident. The pilot stated he was on initial takeoff climb over the lake at about 100 feet and 60 knots, when he noticed a decay in rotor rpm. The engine rpm was okay. He made a left turn towards the shoreline and lowered the collective pitch to increase rotor rpm with negative results. He determined he would be unable to make it back to shore and continued with an autorotation. A deceleration was initiated at 50 feet, initial pitch at 10 feet and cushioning pitch as the helicopter entered the water, and simultaneous left cyclic input. The main rotor blades collided with the water, windscreen, himself and the passenger. The helicopter sank. He released his restraints, assisted the passenger in releasing hers, and exited the helicopter deploying the water wings. They came up on the surface and was assisted to shore by a boater. The engine control unit (ECU) was removed and sent to the Arizona FSDO located in Scottsdale, Arizona, for further examination at the Syntronix Corporation in Tempe, Arizona. The flash memory chip was removed from the computer board which contains the engine operational data and installed in a newly manufactured P/C board, and stored data was downloaded. The test data is recorded in 1 second increments, and the following information was retrieved: "At mark seventeen (0:00:17) seconds the engine start became steady with a stabilized idle. At mark two (2) minutes and fifty seven (0:02:57) seconds the helicopter becomes light on the landing skids. Lift off occurs at (3) minutes and three (0:03:03) seconds. The engine rpm steadily decreases as the percentage of throttle position increases right up until impact, eleven (11) seconds from lift off." The FAA inspector stated, "The pilot apparently applied too much collective pitch too soon and it appears he basically got behind the power curve."( See FAA inspector statement dated May 7, 1998). The helicopter was released to Mr. Olen B. Brock Jr., on April 23, 1998. The ECU was released to Mr. Brock on June 2, 1998.