THE FLT CONDITIONS FOR THE AIR AMBULANCE TRIP CONSISTED OF A CLEAR, MOONLESS, DARK NIGHT AND THE DEST ARPT WAS IN A MTN VALLEY WITH THE ONLY GROUND REFERENCE LIGHTS THE TOWN ADJACENT TO THE ARPT. WITNESSES SAW THE ACFT OVERFLY THE TOWN AND ARPT AT PATTERN ALT THEN HEAD OUT OVER THE LAKE NORTH OF THE ARPT. ABOUT 1 MILE FROM THE RWY, THE ACFT WAS SEEN TO SUDDENLY PITCH UP, ROLL INVERTED AND DIVE STRAIGHT DOWN INTO THE LAKE. THE ACFT WAS HEADING AWAY FROM THE ONLY GND REF LIGHTS AND WAS OVER A REFLECTIVE BODY OF WATER NEAR THE BASE TURN POINT WHEN THE ACCIDENT OCCURRED. WITNESSES HEARD INCREASED ENG/PROP NOISE BEFORE IMPACT. WRECKAGE REVEALED EVIDENCE OF PWR AT IMPACT. THE SHIFTS FOR THE PLTS IN THE OPERATION CONSISTED OF 4 DAYS ON, 2 DAYS OFF, WITH ALTERNATING DAY AND NIGHT SHIFTS. THE PLT WAS ON THE 4TH NIGHT OF THE CURRENT SHIFT CYCLE AND WAS ALSO GIVING FLT INSTRUCTION DURING THE DAYS.