I would personally never trunk it in norcal unless its august and like 85-90 degrees outside. North of point conception, the water temps never break 70 degrees in mid-summer and as you head north, it gets slightly colder as you go. It will be the mid 60s most likely. A 3/2 with no boots is perfect. I wore a 3 mil in august in Santa Cruz and was comfy. I honestly did not see any locals trunking it up there and Santa Cruz is technically central CA. Most days in nor call have marine layers with light breezes. You typically dont feel "hot" anywhere near the pacific up there. You have to go inland a bit.
The water in San Diego last summer never went above 70 degrees. I trunked it zero days last summer. That is not common, but the warmest socal ever gets is like 73 or so... California water is never warm, despite what people tell you. Its never "cold" either... just mild. 55-70 all year.
3/2 and booties. i lived there for a year back in '05. you can ditch the booties if it'll be a shorter session, but most of the spots are pretty rocky, so you're generally better off with something on your feet if only for that reason.