Depends on your level. Most of these are in the summer. The highest winds are starting in March, but if in San Franciso bay, it is very cold in the water.
If you are a beginner, then try Shoreline in Mountain View. Rental Available
Crown Beach in Alemeda is nice. Warm, (mud flats) so shallow and great for learning. You can stand up if you fall in. Rental Available For a shortboarder, ask around the Berkely Sailing Club in the marina.
If you have your own equipment, then there are ALOT OF CHOICES. You may also rent shortboards at ASD surfing in San Mateo.
San Francisco Bay is one of the premeir places in the world for shortboard windsurfing.
If you are an intermediate shorboarder and can water start, again Crown Beach is a good compromise for a starter. from there, I would try Palo Alto Harbor. Flatter water
Next ( and this is what I know) Be aware of the tides!!!
Candlestick park. Flat water sailing. Watch out for incoming tide. It swirls around and drags you out to the Bay.
Next level is Coyote Park. Bump and Jump Ebb tide is the best. Intermediat e shortboarder. You must be able to waterstart.
Highest level in South Bay is 3rd Ave.. Bump and Jump Ebb tide is the best. At lower tides, you can sometimes stand up
In San Francisco Bay, there is Chrissy field. I am not familiar with this, but I think they have rentals and a school there.
Again, be aware of the tides. I recommend wearing a life preserver. You MUST wear a warm wetsuit of at least 3/2 thickness in these areas.