Treasure Island is probably the best and most consistent windsurfing on the San Francisco Bay, ESPECIALLY late in the season. My friends have been religiously going there for years in August and September because when other spots start shutting off or becoming more intermittent, TI becomes the "epicest*". The fetch at Treasure Island is 4 or 5 miles. The swell has that same amount of space to organize, space out, and otherwise become beautiful.
But before you wipe your drool and grab your gear from this description, I must inform you of dear old caveat emptor. Treasure Island has the worst launch of the whole bay. A 25-30 yard u-shaped wind eating jetty from which you launch in the downwind armpit. Launching goes straight downwind in annoying fickle wind. Thus, coming in requires going directly upwind in garbage wind. Also, if you're sailing TI, you'll probably want to be on a pretty small board so you can make the most of the conditions beyond the shadow. Most of us choose to doggy-paddle home at the end of our session, but a handful feel as though they must SAIL their SAILBOARD in. I've never seen a shadow sailor beat a swimmer, but they sure are fun to watch:
The other thing to note is that if something were to go wrong, it could go pretty darn wrong. A couple days ago a few guys went out at TI, then got swallowed up by fog. They all made it home, but they all also had pretty scary experiences finding their way back.(Link to those stories) If your gear breaks down between Treasure Island and Angel Island, it could potentially take you a very long time to get back to dry land in any direction. Not to mention the large tug boat traffic, bridge construction traffic, and 25-knot-traveling commuter ferries.
TI is the best spot in the bay, but ya gotta respect it.
*epicest is a term coined by the German Shepherd for "most epic". Pronounced with a hard "c".