I've sailed for all three
I have worked as a skipper for all three of the charter companies that do day charters. Harbor Sailboats has the best maintained boats and Harbor Island Yacht Club has the largest fleet, at least in one place. Both are on Harbor Island, close to the airport. Seaforth has the worst maintained boats, although usually adequate for a day sail and they have bought some new ones. They rent from at least 3 locations, downtown at the Marriott, in Coronado at Glorietta Bay, and at Mission Bay. Have fun!