THE PLT HAD TRIED TO FLY TO SILVER LAKE, EARLIER THAT MORNING, BUT HAD TO ABORT THE MISSION DUE TO LOW CEILING & VISIBILITY. WHEN HE HAD RETURNED AT 1110 ADT, THE CEILING HAD IMPROVED, BUT THERE WAS STILL LAYERS OF STRATUS & FOG BELOW THE MAIN CLOUD BASE. WITNESSES OBSERVED THE ACFT CIRCLE TWICE & BELIEVED THE PLT INTENDED TO LAND. DURING A DESCENDING LEFT TURN WITH 15 TO 20 DEG OF BANK, THE ACFT STRUCK THE WATER WITH THE LEFT FLOAT, THEN BOUNCED HIGH ABOVE THE WATER. AT THE TOP OF THE BOUNCE, HIGH ENG WAS HEARD, THEN THE ACFT PITCHED DOWN ABRUPTLY & CRASHED IN THE WATER. WHEN THE WATER & SPRAY SUBSIDED, WITNESSES SAW 2 FLOATS, THE REAR SECTION OF THE FUSELAGE & DEBRIS ON THE WATER. ABOUT 2 MIN LATER, THEY SAW THE 2 OCCUPANTS CLIMB ONTO THE FLOATING WRECKAGE. THE WITNESSES BEGAN BUILDING A LOG RAFT TO RESCUE THE OCCUPANTS, BUT BEFORE COMPLETING THE RAFT, THE OCCUPANTS HAD DRIFTED OUT OF SIGHT IN FOG & RAIN. A SEARCH WAS MADE FOR THE OCCUPANTS, BUT THEY COULD NOT BE FOUND & WERE PRESUMED TO HAVE DROWNED.