As an AI language model, I don't have access to real-time data, so I cannot provide the most up-to-date information. However, as of my knowledge cutoff date in September 2021, it is common for swimmers to swim across the San Francisco Bay, including the famous Alcatraz Island swim. However, swimming across the bay and back from San Francisco's shores is a challenging feat due to factors such as cold water temperatures, strong currents, and boat traffic.


The most well-known open water swimming event in San Francisco Bay is the annual "Escape from Alcatraz" triathlon, which includes a 1.5-mile swim from Alcatraz Island to the shores of San Francisco. Many experienced open water swimmers participate in this event, and some swimmers may choose to swim back to Alcatraz from San Francisco, making a round trip. However, this is done as part of a supervised and organized event with support from event organizers, safety boats, and trained lifeguards.


It's important to note that swimming in open water, especially across a busy bay, can be dangerous and should only be attempted by experienced swimmers with proper training, preparation, and safety precautions. Swimmers attempting such swims should be aware of the risks involved and take appropriate measures to ensure their safety, such as wearing wetsuits, using safety equipment, swimming with a buddy or a support crew, and being aware of the current weather and water conditions.