I have swam in the aquatic park in the Marina district of San Francisco, probably the closest direct point to Alcatraz and I'm sure it's more than a mile. My buddy was training for a triathlon and wanted to get some swimming in and I offered to go with him. He had a full wetsuit including hood. I was a junkie at the time and opted for only a thin rubber skull cap and a speedo. Certainly my pain receptors were close to dead, but after about 5 minutes I could tell things weren't functioning correctly. I waved my friend down as asked him to stick with me til I got to shore because I was starting to seize up. Again, this was about 5 minutes in the water. We got to the shore and I was blue and couldn't speak. I mean, I tried to speak but the connection between my brain and my mouth had been frozen. I was like "buh, buh, buh, buh." Just sounds and grunts. I just don't see anybody surviving a swim from Alcatraz to land without a full wetsuit...