At 13:10 Pacific time, a Piper PA-31-350 suffered a collapsed landing gear during a precautionary landing on a beach at the Lake Mead National Recreation Area. The precautionary landing was precipitated by a loss of power in the left engine during cruise flight and the inability of the pilot to maintain altitude on one engine. The pilot of the Part 135 flight and nine passengers were not injured. The pilot said the airplane departed Boulder City at maximum gross takeoff weight and climbed to a cruise altitude of 9,500 feet msl. About 20 minutes into the flight, the pilot heard a loud pop sound and the left engine surged once. The pilot turned around and started back toward lower terrain. The pilot said the airplane would not hold cruise altitude at best single engine rate of climb airspeed and began to drift down. He said the beach was the only clear flat area. During the landing rollout, the aircraft encountered moderately sized rocks and collapsed the nose and left landing gear.