The failure of the instructor pilot and dual student to begin the water takeoff with adequate clearance from the lake shore for the dual student to complete the takeoff and the failure of the instructor pilot to recognize in a timely manner during the takeoff that there was not adequate clearance from the lake shore to complete the takeoff and abort the takeoff, resulting in the airplane colliding with the terrain near the lake shore and coming to rest in a field about 900 feet inland from the lake shore.