What happened
The FAA’s narrative, in sentence case. The FAA writes it in capitals and removes private details.
Pilot flew to second lake near Nanwalek AK intending on landing and then beaching on the west shore. According to the pilot, the water along the shore line is very clear and the bottom is visible. After a couple of touch and go's he landed on the lake and did a slow idle taxi to the beach. About 50 or more feet from the beach he turned the engine off and coasted the remaining distance. He then turned the float plane around and put it's sterns on the beach. He reportedly checked the float compartments at the step and noticed what he reported as minimal water in the left float, he closed the float compartments and then climbed in and after engine start began to idle taxi away from the beach. It took a few minutes to taxi, at idle, about 700 to 1000 feet away from the beach at which point he felt he was FAR enough away from the beach for noise abatement and appropriate to step taxi to the opposite end of the lake. At this time, he applied full power with the yoke fully aft. The airplanes pitch attitude never changed (from idle taxi, to hump/plow, to step) it remained in the idle taxi/displacement attitude. He reportedly knew something was wrong so, he pulled power to idle and then looked out his left window at the left float and noticed water flowing over the top of the float. The shoreline to his right was only a few hundred feet away so, he turned right and tried to make it to shallower water and shoreline. The left float then immediately went further under, more so at the bow. He believed at that point he pulled the mixture to idle cut off, and the front of the airplane went in nose first and the bow seemed to hit bottom because it paused and stood straight up (nose under, tail up/out) long enough for him to egress and make it to the shore line before it flipped completely upside down.
As the FAA filed it
PILOT FLEW TO SECOND LAKE NEAR NANWALEK AK INTENDING ON LANDING AND THEN BEACHING ON THE WEST SHORE. ACCORDING TO THE PILOT, THE WATER ALONG THE SHORE LINE IS VERY CLEAR AND THE BOTTOM IS VISIBLE. AFTER A COUPLE OF TOUCH AND GO'S HE LANDED ON THE LAKE AND DID A SLOW IDLE TAXI TO THE BEACH. ABOUT 50 OR MORE FEET FROM THE BEACH HE TURNED THE ENGINE OFF AND COASTED THE REMAINING DISTANCE. HE THEN TURNED THE FLOAT PLANE AROUND AND PUT IT'S STERNS ON THE BEACH. HE REPORTEDLY CHECKED THE FLOAT COMPARTMENTS AT THE STEP AND NOTICED WHAT HE REPORTED AS MINIMAL WATER IN THE LEFT FLOAT, HE CLOSED THE FLOAT COMPARTMENTS AND THEN CLIMBED IN AND AFTER ENGINE START BEGAN TO IDLE TAXI AWAY FROM THE BEACH. IT TOOK A FEW MINUTES TO TAXI, AT IDLE, ABOUT 700 TO 1000 FEET AWAY FROM THE BEACH AT WHICH POINT HE FELT HE WAS FAR ENOUGH AWAY FROM THE BEACH FOR NOISE ABATEMENT AND APPROPRIATE TO STEP TAXI TO THE OPPOSITE END OF THE LAKE. AT THIS TIME, HE APPLIED FULL POWER WITH THE YOKE FULLY AFT. THE AIRPLANES PITCH ATTITUDE NEVER CHANGED (FROM IDLE TAXI, TO HUMP/PLOW, TO STEP) IT REMAINED IN THE IDLE TAXI/DISPLACEMENT ATTITUDE. HE REPORTEDLY KNEW SOMETHING WAS WRONG SO, HE PULLED POWER TO IDLE AND THEN LOOKED OUT HIS LEFT WINDOW AT THE LEFT FLOAT AND NOTICED WATER FLOWING OVER THE TOP OF THE FLOAT. THE SHORELINE TO HIS RIGHT WAS ONLY A FEW HUNDRED FEET AWAY SO, HE TURNED RIGHT AND TRIED TO MAKE IT TO SHALLOWER WATER AND SHORELINE. THE LEFT FLOAT THEN IMMEDIATELY WENT FURTHER UNDER, MORE SO AT THE BOW. HE BELIEVED AT THAT POINT HE PULLED THE MIXTURE TO IDLE CUT OFF, AND THE FRONT OF THE AIRPLANE WENT IN NOSE FIRST AND THE BOW SEEMED TO HIT BOTTOM BECAUSE IT PAUSED AND STOOD STRAIGHT UP (NOSE UNDER, TAIL UP/OUT) LONG ENOUGH FOR HIM TO EGRESS AND MAKE IT TO THE SHORE LINE BEFORE IT FLIPPED COMPLETELY UPSIDE DOWN.