When John Bate climbs from his 40-foot ketch La Rubia onto her wharf he steps from his home to his hobby. As port director of San Diego, Bate watches from his harbor office during the day and from his boat at night over one of the busiest commercial and Navy harbors in the country. But it is San Diego's 5,000-odd pleasure-boat skippers who have profited most from Bate's affection for anything afloat. For them, and for visiting yachtsmen, he has created Shelter Island yacht harbor, an artificial, concrete-covered sandspit jutting into San Diego Bay. Here the crew of any boat can tie up for a night, take a hot shower, relax in the Polynesian lounge and get navigational help, all free of charge.