The helicopter was being flown to a maintenance facility to correct a 'rotor vibration' that had been detected by the pilot. While in cruise flight over a reservoir, a loss of engine power occurred, when the #2-1/2 bearing failed due to oil starvation. The oil starvation resulted from an obstruction in the bearing oil feed tube by a particle of foreign material. There was evidence the material was introduced into the system during overhaul, about 9 flight hours before the accident. The pilot made an autorotation through dense fog to the surface of the reservoir, which contained patches of ice. The helicopter sank in 190 ft of water. Water temperature was 30 deg F, air temperature was 34 deg, and wind was 170 deg at 8 kts. According to available information, useful conscious time under these conditions would have been about 15 min. The first rescue personnel reached the pilot approximately 45 min following the accident. The pilot was the only person on the surface at that time.