THE PILOT HAD A PRIVATE, SINGLE ENGINE LAND RATING AND WAS ENROUTE TO TAKE HIS SINGLE ENGINE SEA FLIGHT CHECK. HE DELAYED AT A LAKE THREE MILES FROM HIS DESTINATION TO PRACTICE SOME TAKEOFFS AND LANDINGS. WHILE ATTEMPTING A POWER OFF LANDING HE GOT SLOW AND HIGH ON FINAL AND DID NOT ADD POWER IN TIME TO STOP THE SINK RATE RESULTING IN A HARD LANDING.