The certificated airline transport pilot was receiving instruction for a seaplane rating. He made three water landings in a float-equipped seaplane, and after the third landing, the flight instructor told him to climb to and maintain 100 feet. The pilot climbed the airplane to the assigned altitude, and made a left turn to a downwind leg at 80-85 knots. Once on the downwind leg, the instructor simulated an engine failure by pulling the power lever back to idle, and the pilot receiving instruction began a left turn to land into the wind. The instructor then told him to turn to the right, and subsequently joined him on the controls for the right turn. Neither pilot added power, and the airplane "landed hard," in a descending right turn, at an estimated 45-90 degrees from the wind line. Upon landing, the left float separated from the airplane, and the airplane subsequently sank. No mechanical anomalies were noted.