THE ATP PLT WAS PRACTICING 'SPLASH AND GO LANDINGS' ON LAKE POWELL WITH TWO PSGRS. HE LANDED TO THE WEST IN WINDS HE ESTIMATED WERE LESS THAN 5 KTS. HE SAID THAT DURING THE LANDING RUN, THE ACFT STRUCK AN UNSEEN SUBMERGED OBJECT, POSSIBLY A LOG FLOATING BENEATH THE SURFACE; THE ACFT SANK IN 130 FT OF WATER. THE OBJECT PENETRATED THE HULL BETWEEN THE RUDDER PEDALS AND BROKE BOTH OF THE PLT'S LEGS. HE SAID THE IMPACT SEPARATED THE CABIN FROM THE EMPENNAGE. THE FRONT SEAT PSGR WAS EJECTED THRU THE WINDSHIELD ALONG WITH THE SEAT, SEAT BELT & SHOULDER HARNESS ATTACHED. WITNESSES REPORTED THAT THE ACFT NOSED OVER AND CAME TO REST INVERTED IN THE WATER. ALL 3 OCCUPANTS WERE RESCUED BY A PASSING SKI BOAT. THE PLT WAS AIR LIFTED TO PAGE, ARIZONA, FOR EMERGENCY TREATMENT. A POST-SALVAGE INVESTIGATION VERIFIED HULL PENETRATION IN THE AREA OF THE PLT'S FEET.