Of all the Californian animals that occasionally attack people, great white sharks are likely the scariest. Say what you will about pumas, bears, and coyotes: none of them grow 20 feet long or attack you at high speed from the unseen depths of the ocean. Nightmare fuel indeed. Still, only one or two shark attacks a year are recorded in California, with a death about once a decade. Considering the fact that millions of people play in the Californian surf during shark season, that says something about just how unlikely shark attacks are.
Part of the reason is that there are a lot fewer great white sharks around California than there used to be: only a bit more than 200 individual great whites were found in a recent census. (There are 34 or so species of sharks off the Pacific Coast of North America, and most of them are suspected to have attacked humans at one point or another, but all recorded attacks in California involve the great white shark.)
Even when there were more great whites in the water here, attacks on people were rare. We are not the great whites' preferred food. The fish evolved to prey on the abundant population of marine mammals here on the Pacific Coast: seals, sea lions, cetaceans, and sea otters. Young great whites will eat large fish, but once they've grown past about 14 feet in length great whites strongly prefer marine mammals for their meals. Most recorded attacks in California are thought to occur when the sharks mistake people for pinnipeds: attacks usually involve one bite, at which point the shark presumably realizes its mistake.
Still, not everyone is as lucky with that one bite as this kayaker was last month off the Cambria coast. One bite can ruin your day, especially if it severs a major artery. The majority of deaths from shark attack in California seem to involve blood loss. That bite may be over within seconds with absolutely no prior warning. Preventing shark encounters thus becomes the most prudent course of action. Though the canonical shark-danger area is in Northern California's "red triangle" between Mendocino, Monterey and the Farallon Islands, great white attacks have occurred in every coastal California county save Ventura. Avoiding shark bites is thus good practice from San Diego to Crescent City.
This involves accounting for shark behavior by changing your own. Sharks are attracted to areas with lots of marine mammals, for instance, and so if you see a few dozen sea lions frolicking in the surf when you hit the beach that's a strong hint that Jaws may be lurking a few yards beneath them. Sharks seem to prefer murky water, and have been known to attack out of the depths near steep drop-offs, so keep to the clear water and away from the edges of abysses. And stay out of the water during dawn, twilight, and nighttime: sharks use poor visibility to their advantage.
Despite grown sharks' preference for marine mammals, they're still attracted to fish and fish parts. Shark tourism operators attract giant great whites by "chumming," or dumping buckets of fish guts into the sea. You might thus deem it wise to avoid swimming in areas with lots of bait angler activity, where fishermen and women have been tossing their old bait buckets into the surf. Similarly, areas with lots of active fish-eating birds diving into the water and catching meals are probably good to avoid. As sharks tend to pick off marine mammals who are separated from their group, sticking with your friends as you swim, surf, dive or kayak will likely reduce your attractiveness to a passing shark.
There are a few other shark behavioral triggers worth noting. Thrashing in the water attacts sharks, so when you do swim swim calmly and surely; avoid horseplay and keep your frisky dog to the shallows. Shiny things may do to sharks what metal lures do to bass, so don't swim with jewelry or bright clothing. The thing about sharks sensing blood in the water is true, so avoid swimming if you're likely to put blood in the water. (This proscription would technically include menstruating women, though the jury is still out as to whether the increase in risk during menstruation is worth worrying about.)
And always, always respect and heed instructions from lifeguards or local authorities when they close a beach. Sharks can attack in as little as two feet of water: if the water reaches higher than your shins, you run the risk of losing those shins.
A shark encounter may be more drawn out than the usual sudden ambush bite. In fact, most shark encounters happen with the human involved never finding out. If you do see a shark while diving, kayaking, surfing, or swimming, don't panic. Move deliberately and calmly -- first to join the closest group of people, and then to get out of the water. On rare occasions when a shark will harass a kayaker or surfer, a good whap on the snout usually serves to dissuade them from bothering you further. (Obviously, it's better to use a kayak paddle than your fist to deliver this whap.) The idea isn't to hurt the shark, but rather to send a message: you object to being bothered.
And keep in mind that as with so many other creatures, sharks suffer far greater losses at our hands than we do from them.