DAMAGED ACFT AS A RESULT OF A HARD LNDG.

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ALTHOUGH I HAD A SEAPLANE RATING; I HAD NEVER FLOWN A LAKE AND JUST WASN'T FAMILIAR WITH IT. I THEN HAD HIM TAXI BACK TO RWY 21 AND ADVISED HIM TO DEMONSTRATE A SHORT FIELD OR MAX PERFORMANCE TKOF AND LNDG. HIS TKOF WAS EXCELLENT AND; ON DOWNWIND; I ADVISED HIM TO IMAGINE A 50 FT TREE BEING RIGHT AT THE END OF THE RWY AND HE WOULD NEED TO CROSS THE END OF THE RWY AT APPROX 100 FT AGL. HIS APCH WAS EXCELLENT; VERY STABLE; WITH A CONSTANT AIRSPD SOMEWHERE AROUND 80 KIAS OR SO I THINK; WITH A CONSTANT RATE OF DSCNT. GEAR AND FLAPS WERE DOWN; AND HE CROSSED THE END OF THE RWY IN EXCELLENT OR PERFECT POS; ABOUT 100 FT AGL. BUT IMMEDIATELY AFTER XING THE RWY; HE THEN PUT THE PLANE IN WHAT I WOULD CALL AN EXCESSIVE ANGLE OF DSCNT. HE REALLY CAUGHT ME BY SURPRISE; BECAUSE I WASN'T EXPECTING THIS LESS THAN 100 FT OFF THE GND. I IMMEDIATELY GRABBED THE WHEEL AND PULLED BACK BUT IT WAS TOO LATE TO AVOID CONTACT WITH THE RWY. THE ACFT HIT IN A FLAT ATTITUDE; BUT IT HIT HARD; BREAKING OFF THE L MAIN AND DAMAGING THE NOSE GEAR. IT ALSO WRINKLED THE SKIN ON THE WINGS. I HAD NEVER MET OR FLOWN WITH XY BEFORE; SO I DON'T KNOW IF HE MADE ALL OF HIS APCHS LIKE THIS OR MAYBE THIS IS SOMETHING HE PICKED UP FROM THE LAKE REPRESENTATIVE.