Hello vilne3,
Being that the great white shark is the world's largest predatory fish, growing to an average size of 15 feet (the largest recorded specimen was 21 feet) it can pretty much go where ever it darn well feels like going, hehe, but is very rare in Southern California. It ranges worldwide in temperate and subtropical seas, but sightings in general are usually considered pretty uncommon. When I say Southern California, I am basically referring to the likes of San Diego (where we are located) and surrounding areas.
Now, keeping that in mind, the waters off central California offer a healthy source of food for great whites, and every summer and fall they actively feed in those areas. The Farallon Islands, a national wildlife refuge (about 27 miles off San Francisco), is a common feeding ground for the sharks. Seals and sea lions live around and breed on these islands, providing the great whites with a rather large "Grocery Store", if you will. Another common feeding area for the sharks are the coastal waters along central Californias Ano Nuevo State Reserve and along the Marin Headlands. In the summer the sharks feed on seals and sea lions along the coast as far north as Oregon and every now and then the Gulf of Alaska. In the Fall, they turn south and feed along the offshore islands. It is believed (still not known) that female Great Whites migrate to southern California to give live birth to their offspring. In abnormally warm-water (El Niño) years, they are more plentiful off central California because both the sharks and their meals, are shifted north.
In more than a decade of diving Southern California I have never seen a Great White here. Studies show that population of Great Whites off California are pretty low, with probably less than 100 adult total.
More and more "Shark Watching" trips have started to pop up since in 1992 they were placed on the protected species list for the state of California due to their importance to our marine ecosystems.
Let me know if this helps, and if you had a specific place in mind I will see if I can't dig some info up for you.
=-)
PS.
Interesting Fact: More people are killed in the U.S. each year by dogs than have been killed by white sharks in the last 100 years.