The pilot stated that he was demonstrating stalls to his son. He said that he had recovered from the first stall without incident and climbed to 2000 feet. He said he initiated a second stall (over the area of a lake). Also, he said that while attempting to recover, the engine failed to return to full power as the throttle was advanced, and the airplane entered a spin; then while recovering, the right wing tip struck the water, and the airplane cartwheeled and sank.