The sport pilot reported that he was landing his float-equipped, experimental amateur-built airplane on a remote, smooth and glassy water lake. The pilot said that during the approach, just before touchdown, the airplane encountered a slight downdraft, and the airplane's right float struck the water, which pivoted it to the right. The left wing subsequently struck the water, and the airplane nosed over and sank. The pilot reported no mechanical anomalies that would have precluded normal operations.