With the exception of Leatherback Turtles which have a more northerly distribution and are observed swimming and feeding offshore in this area occasionally, most other sea turtle sightings in this part of California are rare and involve dead stranded animals.
I think the turtle in this photograph is an Olive Ridley Sea Turtle (Lepidochelys olivacea). It's carapace was ~62 cm long. Although I'm not completely familiar with their status in this region, it sounds like there have been at least 4 local records since the late 1980s: one at McClure's Beach on Point Reyes in January 1987, one offshore of Marin County in October 2001, one in Tomales Bay in November 2002, and one at Stinson Beach in November 2009.
There's a short article about central California sightings in this Marine Turtle Newsletter. And more general information about Olive Ridleys is available on this National Marine Fisheries Service web page.
If you happen to find a stranded sea turtle, it's best not to move it, but to contact stranding experts at either the California Academy of Sciences (415-379-5381) or The Marine Mammal Center (415-289-7350).