On September 8, 1996, at 1448 Alaska daylight time, a wheel equipped Piper PA-18 airplane, N4372A, registered to and operated by the Civil Air Patrol, Arcturus Wing, Anchorage, Alaska, lost engine power during the initial climb after takeoff while towing a glider. A forced landing in a lake was accomplished. The U.S. goverment operated flight was operating under 14 CFR Part 91, and was departing Elmendorf Air Force Base Runway 33 for multiple glider tow operations. Visual meteorological conditions prevailed and a visual flight rules flight plan was in effect. The commercial certificated instructor pilot (pilot-in-command), seated in the rear seat, and the commercial pilot seated in the front seat, were not injured. The airplane sank in a lake and was substantially damaged. The purpose of the flight was to provide aerial tows to gliders, and to instruct the commercial certificated pilot (dual student) in glider tow operations. During an interview with the instructor pilot on September 12, 1996, he stated that they had completed two glider tow operations while he was flying from the front seat. The pilots then exchanged seats. The dual student exited the airplane, and the instructor pilot climbed over the front seat to the rear seat. This was accomplished while the engine was running. They completed one more successful tow operation, and on the fourth tow, when they reached an altitude of 200 to 300 feet above ground level, the engine lost power. They selected a muskeg field in which to land, but at tree height the engine surged to full power for approximately 2 to 3 seconds and again lost power. They missed the field and had to land in the lake. Examination of the airplane cockpit prior to removal from the lake showed that the fuel selector handle indicator was pointing between the left and right fuel tank positions. During the interview with the instructor pilot on September 8, 1997, he was asked about his and the dual student's emergency actions upon experiencing the power failure. He was asked directly if they had attempted to reselect a fuel tank and he stated "no." Postaccident inspection of the airplane and its systems disclosed no evidence of any preimpact mechanical anomalies. The old, lake water-contaminated fuel was removed, and fresh fuel added. The engine was started and ran normally. The fuel selector was positioned as found when the airplane was submerged in the lake. The engine ran for approximately two minutes and stopped.