We did the same cruise in reverse. Flew to Hawaii and cruised the islands and then four days to Ensenada. From Ensenada we got on a bus that took us to the airport in San Diego where most of the people had flights but we took a cab to the train station and took the train to the Fullerton station and had a friend pick us up there.
I would guess that RCL has a bus from the San Diego airport to Ensenada. It was not too expensive. Have a fun cruise (you will) Don