Well its a lot warmer down south, LOTS warmer. But its frigid here. You have hypothermia inside of a few minutes without a wet suit on. Surfers go in the water, but only in wet suits, same with the windsurfers. And if you really want to swim, there is the Bay down at the park, which is nice, sheltered - but still cold. However you get to swim around old sailing ships, so that's kind of cool. And there are people there swimming all the time. But really, its Cali, and in SF there are few pools, everywhere else, there are tons. My northern apartment has a year round heated pool, and two six person hot tub/spa things, and I go the Russian River a couple times a summer, so there is the option.
The real problem at most beaches up north is the rip-currents, undertows and sleeper waves. We have quite a few people drown every year. They get caught in the current and get washed out to sea.
