Although I live near SF I've never searched for Hobie rentals, nor am I aware of any rentals.
If you do manage to find a boat, then sailing the SF bay can be a real challenge depending on your launch site. In front of the Golden Gate Bridge high winds, strong currents, and cold water require a wetsuit and strong seamanship. Also, there is only one beach in SF inside the bay from which to launch - Crissy Field, practically under the Golden Gate Bridge.
You could also try contacting the Hobie fleet in the north bay for a schedule of activities - http://www.hobiefleet281.com/
By the way, the city of San Francisco in summer can be cold and foggy in the afternoon, so if you stay there then bring something warm to wear in the evenings. However, if you're staying in Napa then the weather can be HOT.