The pilot of the amphibious float-equipped airplane reported that, after the owner had completed multiple touch-and-go water landings, he elected to take the flight controls and perform a few water landings himself. He added that his first touch-and-go water landing "went very well." However, during the second touch-and-go water landing, the touchdown was smooth, but as he added power to "begin the climb away", the airplane veered right and nosed over into the water. He added that, the right float may have struck a submerged object during the landing. The right wing lift strut sustained substantial damage. The pilot reported that there were no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation. According to a Federal Aviation Administration (FAA) Aviation Safety Inspector who inspected the accident airplane postaccident, the forward first third section of the right float was bent upward and had "scratching and scoring" marks on the bottom side of the float. He added that he did not observe any rust or corrosion on either float. A review of the FAA airman certification database revealed that neither pilot held an airplane single engine sea rating.