Good morning Michelle,
Apologies for the delay in replying, I have been awaiting a reply from our Tour Manager based out in California. you are advised to pre book Alcatraz via the Internet. Ideally after the San Francisco city tour, so 2:30pm or later, if you plan on doing the Bay Cruise. If your not going on the Bay Cruise, you can book for after 1:00pm.
I hope this helps,
Kindest,
Ben