The pilot/mechanic completed an annual inspection on the aircraft and intended to perform a short test flight out over the channel between the mainland and an island. The ATCT cleared the aircraft for takeoff at 1757 hours with a right downwind departure. There were no documented radio contacts with the aircraft after the pilot left the local control frequency. Recorded radar data showed the aircraft departing the Torrance Airport traffic area and proceeding south bound over the channel for a distance of about 10 miles. The target then reversed course and was headed for the mainland again when the target track appeared to make an abrupt left turn and disappear at a point about six miles south of San Pedro. Several witnesses were in boats in the channel at about the time the target disappeared, and in the general area of the last recorded radar position. Several of the witnesses reported hearing sounds of an engine sputtering then the sounds of a large splash. One witness reported observing a low flying aircraft at about 1830 hours just prior to hearing the large splash sounds. The witness could not, however, identify the aircraft make, model or even specific class (single or multi engine) that he observed.