On April 14, 1999, about 1129 central daylight time, a Lake LA-4-200, N959DF, registered to an individual, ran off the runway while landing at Pensacola Regional Airport, Pensacola, Florida, while on a Title 14 CFR Part 91 instructional flight. Visual meteorological conditions prevailed at the time and no flight plan was filed. The aircraft received substantial damage and the airline transport-rated pilot and commercial-rated dual student were not injured. The flight originated from Pensacola, Florida, the same day, about 0945. The instructor stated that while practicing water landings they noticed the electrical power was at 10 volts and the electrical system was discharging. They terminated the flight training and contacted Air Traffic Control for a return to Pensacola Regional Airport. They were cleared for a straight in approach to runway 8. The hydraulic pressure was showing 700-800 pounds and they lowered the landing gear. The main landing gear appeared to lock down; however, the nose landing gear did not lock. The instructor took control of the aircraft and informed the ATC controller that he would bounce the aircraft in an attempt to lock the nose landing gear. Upon touchdown the nose gear did lock, but the aircraft began to drift to the left. Attempts to correct the left drift were unsuccessful and the aircraft ran off the runway to the left, hit a runway sign, and stopped after a 160/170-degree turn to the left. He believes the left drift was caused by the left main landing gear collapsing during landing roll out. Postcrash examination of the aircraft and runway was conducted by an FAA inspector. A visual inspection of the runway did not reveal any signs of aircraft parts or heavy tire marks. Examination of the aircraft showed the left main landing gear had come through the top of the left wing. The nose of the aircraft was damaged due to hitting a runway sign. (See FAA Inspector Statement)