The pilot stated that he was flying the weight-shift control aircraft over a river while the passenger took photos. As he slowed and descended the aircraft near a bird's nest in order for the passenger to capture a photo, the aircraft entered an aerodynamic stall. The pilot applied full engine power to recover, but the aircraft continued to sink before impacting the river, resulting in substantial damage. The pilot and passenger subsequently egressed and swam to the shore. The pilot stated that there were no mechanical malfunctions or anomalies of the aircraft or engine that would have precluded normal operation of the airplane.