The 28-year-old Na finished at 23-under-par 261. He broke the tournament's scoring record of 263, which had been posted four times previously.
Watney, who shared the third-round lead with Na, closed with a four-under 67 at TPC Summerlin to end alone in second at minus-21. Watney was shooting to become the first three-time winner of the season on the PGA Tour.
Paul Goydos and Tommy Gainey both shot three-under 68s Sunday and shared third place at 18-under-par 266. Tim Herron (69), David Hearn (65), Carl Pettersson (68), Jhonattan Vegas (68) and Spencer Levin (68) were one stroke further back at minus-17.
MORE TO FOLLOW.