The pilot reported that, following an uneventful water landing on a lake, the amphibious airplane was slowing down and transitioning from the step when he heard a loud noise followed by an immediate yaw to the right. Subsequently the right wing struck the water, which was followed by the pilot applying power in an attempt to raise the wing. However, the pilot was unsuccessful and elected to turn off the engine and ordered his passengers to exit the airplane. Shortly after the pilot exited the airplane, he observed it roll over upside down in the water and noticed that the right wing float was mostly separated from the wing structure. The pilot stated that he felt that the airplane had struck a floating log or some other debris in the lake during the landing. He further stated that there were no mechanical anomalies with the airplane that would have precluded normal operation.