THE PILOT STATED THAT JUST AFTER TOUCHING DOWN ON THE WATER, HE ENCOUNTERED A MICROBURST. HE ESTIMATED THAT THE WIND SPEED SUDDENLY CHANGED FROM ABOUT 15 TO 20 KNOTS TO 40 TO 50 KNOTS. THE LEFT WING FLOAT HIT THE WATER AND WAS TORN OFF. BECAUSE OF THE STRONG WINDS AND ROUGH WATER HE WAS UNABLE TO GET AIRBORNE OR TO TURN THE AIRPLANE. HE KEPT THE AIRPLANE HEADED INTO THE WIND AND BEACHED THE AIRPLANE ON THE OPPOSITE SIDE OF THE BAY. THE PILOT ALSO SAID THAT SUITABLE WEATHER CONDITIONS HAD BEEN REPORTED FOR THE LANDING AREA ABOUT ONE HALF HOUR EARLIER.