After departing North Island NAS (Naval Air Station), the plt attempted to change frequencies, but the megahertz (MHz) tuning knob came off at a setting of 122.1 MHz. He could still use the knob for tuning tenths of a MHz, so he contacted San Diego FSS on 122.1 MHz & advised of his situation. He was concerned about violating the Mexican border & nearby control areas & was concerned about heavy Navy jet traffic at the NAS as well. Therefore, the plt asked the FSS to obtain clearance for him to return to North Island for landing. In the meantime, the plt flew out over the ocean to wait for a clearance. When the clearance was received, he turned the acft to proceed toward North Island & began his prelanding checks. He rolled out of the turn with the gear down & the landing light on. He felt certain that he was at about 1000 ft above the surface, but at about that time, the acft contacted the water. The occupants egressed from the acft before it sank & were picked up by a fishing boat, hrs later. There was fog, haze & 2 mi vis SW of North Island.