Probably the most people I talk to about the Alcatraz triathlon ask this question, "aren't there sharks in the water?" It's the first time I've participated in a triathlon were an infomail was sent out with a specific section on sharks. So the answer is, yes!
Yes, there is marine life in San Francisco Bay including seals, sea lions and sharks. The half dozen different types of sharks within San Francisco Bay are usually small, three to four feet in length, and hang out well below the surface of the water and have no interest in Escape participants. Larger sharks do live outside of the Bay where the water is saltier. Sharks of any significant size do not generally come into the Bay because, especially this year, of the brackish water composition (half salt and half fresh water runoff). The San Francisco Bay has 500 miles of Sierra snow melt running into it, so between the fresh water and the silt, the big sharks usually stay away from the Bay's waters. Harbor seals and sea lions may come by to see what swimmers are up to due to their curious temperaments, but the sheer numbers of swimmers, lifeguard vessels, and support boats keep them at a distance.
From the Escape from Alcatraz infomail.
I wouldn't worry too much, and I must admit that I am more curious about how strong the current is at the given day, as it is said to vary greatly and that it is almost impossible to predict.