There is no food service available on Alcatraz Island - only water.  There are drinking fountains located by both restrooms - at the dock and near the lighthouse, and bottled water is available in the gift shops on the island.

There is a snack bar on the ferry.

Eating, drinking and smoking is only permitted on the dock. Beyond the dock only bottled water in plastic bottles is allowed to be consumed. 

Please refrain from feeding the birds on Alcatraz - as tempting as it may be (the birds will beg and even try to snatch food out of your hands) human food is NOT good for them, dependence on human food can be the death of a bird.