On July 23, 2006, about 1215 Alaska daylight time, a float-equipped de Havilland DHC-2 airplane, N5146G, sustained substantial damage when it collided with the shoreline and trees while performing a step turn/taxi on Seven Mile Lake, about 6 miles south-southwest of Big Lake, Alaska. The airplane was being operated as a visual flight rules (VFR) local area proficiency/instructional flight under Title 14, CFR Part 91, when the accident occurred. The airplane was operated by the Civil Air Patrol Inc. (CAP), Anchorage, Alaska. The first pilot, seated in the right seat, and the second pilot, seated in the left seat, both commercial certificated flight instructors, were not injured. Visual meteorological conditions prevailed, and CAP flight following procedures were in effect. The flight originated at the Lake Hood Seaplane Base, Anchorage, about 1015. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on July 23, the flight instructor reported that the flight was intended to familiarize the second pilot with the de Havilland DHC-2 airplane. After landing on the lake, the second pilot practiced making step turns. The first pilot said that he took the flight controls to demonstrate continuous water taxiing/turning while on-step. He said he taxied around the perimeter of the lake and began a right turn, still on-step, but the airplane did not respond. Application of right rudder did not prevent the airplane from colliding with the lake shore and trees. The airplane received damage to the left wing and fuselage. The first pilot indicated that the wind was about 6 knots from the north-northeast, but he thought the airplane encountered a wind shear at the narrowest portion of the lake where the accident occurred.