On August 20, 1993, at 2030 eastern daylight time, a Bellanca 17- 30A, N104CB collided with the water approximately four miles north of Hilton Head Island, South Carolina. The private pilot and his passenger both received minor injuries while the aircraft was not recovered. The aircraft was being operated under 14 CFR Part 91 by the pilot. Visual meteorological conditions existed at the time, and an instrument flight rules flight plan was in effect for the personal flight. The flight departed Rock Hill, South Carolina, about 1945. The pilot reported refueling the airplane at the last departure point. The pilot stated that as he approached Hilton Head Island, the aircraft engine lost power and he was unable to maintain altitude. He declared an emergency, and ditched the aircraft in the water. He was rescued by personnel from the U.S. Coast Guard, and his passenger swam to shore. The airplane was not recovered; therefore, the reason for the power loss was not determined.