On July 24, 2003, approximately 1900 Pacific daylight time, a Hughes 269C helicopter, N500HS, impacted the waters of Olson Lake, which is located about five miles north of Marblemount, Washington, during an attempted forced landing. The commercial pilot and his passenger were not injured, but the aircraft, which is owned and operated by the pilot, sustained substantial damage. The 14 CFR Part 91 personal pleasure flight, which departed Skagit County Airport about one hour earlier, was being operated in visual meteorological conditions. No flight plan had been filed. According to the pilot, once he arrived in the area of the lake, he made two high reconnaissance passes in order to look the area over before descending to a lower level so he could take a closer look at possible camping sites along the lake's edge. The pilot completed a systems flow check prior to initiating a hover taxi over the lake, and then once the hover was established, the oil pressure and oil temperature was rechecked. The pilot then began "hovering down the middle of the lake," looking at possible landing sites, but ultimately, when he reached the far end of the lake, he decided that the best spot was at the other end where he had first passed over the shoreline. Just about the time he was getting ready to turn back, he noticed a gradual drop in engine rpm's and the aircraft began to slowly descend toward the surface of the lake. Although he increased the throttle, he was unable to increase or stabilize the rpm, and the helicopter eventually settled into the water in a level and upright position. The rpm had initially been 3,200, but by the time the aircraft touched the surface of the water it had dropped to 2,400. Ultimately the aircraft came to rest upright on its skids, in about six to eight feet of water, near the shoreline of the lake. Although the airframe was undamaged, the main rotor blades where damaged when they came in contact with the surface of the water. As part of the investigative process, the older left magneto was removed from the aircraft and tested at various rpm’s (the right magneto was newly installed, and not removed for testing). During the subject testing, the left magneto performed erratically, and did not produce a consistent continuous spark at high rpm's. After the testing was terminated, the magneto was disassembled and inspected, but the specific reason for its inconsistent performance was not able to be determined.