DURING CRUISE FLIGHT AT 500 FT OVER GULF WATERS, THE LEFT ENGINE SUSTAINED A MASSIVE, UNCONTAINED, EXPLOSIVE FAILURE. SHARPNEL PENETRATED THE RIGHT ENGINE, WHICH FAILED FROM DAMAGE. SHARPNEL ALSO PENETRATED THE AC AND DC JUNCTION BOXES, CAUSING COMPLETE ELECTRICAL FAILURE (NO MAYDAY CALL POSSIBLE). SHARPNEL FURTHER SEVERED THE TAIL ROTOR DRIVE SHAFT AND PENETRATED THE ACFT FUEL SYSTEM; FIRE BROKE OUT IN THE TRANSMISSION AREA AND THE COCKPIT FILLED WITH SMOKE. USING THE CO-PLT'S SIDE WINDOW FOR VISUAL ACQUISITION OF THE WATER SURFACE, A SUCCESSFUL AUTOROTATION WAS PERFORMED. HOWEVER, THE ACFT ROLLED OVER AND SANK SINCE THE EMERGENCY FLOATATION GEAR IS ELECTRICALLY OPERATED AND FAILED TO DEPLOY. AFTER SUCCESSFUL EVACUATION BY ALL OCCUPANTS, PLT RETURNED TO THE INVERTED ACFT AND DEPLOYED LIFE RAFTS. INVESTIGATION REVEALED LEFT ENGINE COMPRESSOR-TO-TURBINE COUPLING HAD FAILED IN FATIGUE. TURBINE WENT OVERSPEED AND BURST UNDER CENTRIFUGAL LOADS.