San Francisco is not Los Angeles. The beaches here tend to be chilly, windy, and often shrouded in fog. The water is cold enough that wetsuits are standard for anyone going in past their ankles. But that should not deter you -- the coastal scenery is spectacular, with views of the Marin Headlands, the Golden Gate Bridge, and the rugged Pacific coastline.
Ocean Beach is the big one, stretching five miles along the city's western edge. The smaller cove beaches like Baker Beach and China Beach offer more intimate settings and dramatic clifftop views. East Beach at Crissy Field is the only beach on the Bay side, calmer and family-friendly, and Land's End Beach is really a hike along bluffs with a rocky strand at the bottom.