Yes, it’s true: the water that surrounds San Francisco is cold. Even in July, or come September when it feels like the Pacific should have had all the time it needs to come up to a reasonable temperature. To escape a cobalt-blue ocean that looks pretty but feels like Nova Scotia, San Francisco has a number of on-land options for doing laps. Just keep in mind that all of the city’s pools close at staggered intervals during the winter, sometimes for a month or more.
Don't know how to swim? Don't put it off. As daunting as it seems, swimming lessons are a fulfilling experience with a lot of benefits.