All the swims are AWAY from Alcatraz. Think about it. If you swim TO Alcatraz, and it's a hair over 1.5 miles, when you arrive, wouldn't you be cold and in need of hot drink, towell, and a rest?
That doesn't exist THERE.
If you jump off a boat and swim to AquaticPark, hot drink, towells, and hot showers are awaiting.
I already said I'm a weak swimmer. Longest swims are outside Sloat on a 17' day, to shore, which took maybe 1/2 hour total.