A Cessna 182Q collided with an unmarked 50 ft high cable over the Colorado River. The aircraft was observed flying below the level of the cable, rocking its wings to boaters and sunbathers. One witness stated that the aircraft 'buzzed us, so low, I actually ducked, thinking he would hit us.' The aircraft then started a climb, at which time it collided with the cable. The cable is part of a gage station which measures the height of the Colorado River. The cable spans 548.8 ft between the river banks and is 1 1/8 inches in diameter and constructed with 37 galvanized plow steel strands. The cable crossing is not marked on the Phoenix sectional chart, but the crossing falls within the boundaries of a National Park Service area. Pilots are requested to maintain a minimum altitude of 2,000 ft above the surface of these areas. In addition, FAR 91.79 restricts the operation of an aircraft to no closer than 500 ft to any person, vessel, vehicle, or structure.