Chris,
Check out the website of the only windsurf shop in N.CAL :
www.diabolofunboard.com/Heaps of info/news/links including weather reports (in French though). You will find info on gear rental by clicking on "LOCATION" (means HIRE in French).
Anse Vata is the most popular and accessible spot (slalom-free ride)especially if you stay in one of the hotels right across the road.
Cheers
JMM