According to the flight instructor, the amphibious airplane departed Paris Henry County airport en route to a portion of Kentucky Lake called “Big Sandy." The flight instructor was giving dual instruction to endorse the pilot for a light sport aircraft sea rating. The flight instructor reported that during the initial landing on the lake, as the water line moved about three-quarters of the way up the floats, the airplane suddenly pitched over. The airplane became inverted with the floats on the surface. The two pilots exited the airplane with no injuries and held on the floats until help arrived. The pilot receiving instruction believed the airplane struck an object during landing. A post-accident examination by a Federal Aviation Administration inspector revealed that the right float sustained substantial damage consistent with contact with an object and that no mechanical malfunctions or anomalies were found. The object struck by the seaplane could not be identified.