THE PLT HAD FLOWN TO LITTLE SISSABAGAMA LAKE WHERE HE OWNED A CABIN. A WITNESS HAD OBSERVED THE LAST 3 MINS OF FLT BUT NOT THE ACTUAL IMPACT WITH THE WATER. THE ACFT WAS LATER FOUND INVERTED IN 15 FT OF WATER 300 FT FROM A SHORE. PLT WAS FOUND 150 FT FROM THE SHORE & THE CAUSE OF DEATH WAS DROWNING. THE WITNESS OBSERVED THE ACFT IN A WIDE GRADUALLY DESCENDING LEFT TURN AROUND AN ISLAND. THE ENG SOUND WAS 'NORMALLY LOUD. SHORTLY THEREAFTER, THE ENG SOUND DECREASED MOMENTARILY, THEN STOP. THE LEFT WING FLOAT HAD SEPARATED FROM THE WING. THE HORIZONTAL STABILIZER TRIM TABS WERE ABOUT 3 DEG NOSE UP, A NEAR-CRUISE SETTING. ACCORDING TO THE CFI WHO HAD CHECKED OUT THE PLT IN THE ACFT, THE PLT, WHO WAS A B-727 AIRLINE CAPTAIN, HAD DIFFICULTY JUDGING SURFACE WIND DIRECTION FOLLOWING A LOW PASS OVER THE WATER.' WINDS IN THE AREA ARE UNPREDICTABLE DUE TO THE SEVERAL ISLANDS ON THE LAKE. IT WAS RAINING AT THE TIME & WAVE HEIGHT WAS ABOUT 1 FT. THE WATER TEMP WAS 42 DEG F.