If you do not have tickets for alcatraz it is possible to purchase tickets for the same day if you turn up at around 5am and wait in line till the ticket opens at 7.30 am.
Around 100 tickets are sold then , usually for the first ferry at 8.45am, on a first come first served basis!
I have just returned from the island yesterday (18 th September) and had a great time!
Tickets cost 30 dollars each- much cheaper than the various touts around the bay!