Alcatraz is actually even closer to shore than it looks in pictures. A reasonable question is: why couldn't someone just swim away? Wikipedia claims (and the guidebook agrees) that the icy water ruled out that escape route.
The first thing we saw when we got to the shore was a guy going for a swim. The guy stayed in the water for about forty minutes. But of course you have to allow for the current, which isn't present right by the shore. In the entire history of the prison, there was only one successful (and documented) attempt to "swim away" from Alcatraz. But by the time the inmate reached shore, he was so exhausted that he could only lie there quietly, where the police—called by concerned passersby—found him. A couple of others are still listed as missing. It's entirely possible the escapees survived, but they reached shore not by swimming, but in a homemade boat.
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