I can't really add too much to the already good replies, but I will say that Sonoma diving is worth the effort!! I like to camp at Stillwater Cove campground and have been doing quite a bit of Abalone diving up there this year. We usually bring tanks and scuba in the morning, checking out all the good spots and then go back in the early afternoon and get a few abs.
As for scuba, I think Gerstle is one of the best areas with fairly easy access(if you dont mind slippery rocks). There are lots of good animals to see, since it is protected. If you swim pretty far out, there is a huge reef with lots of Metridiums and cobalt encrusted sponge...it's an awesome sight and gets down to 50 feet or so.
Stillwater is pretty much the same, a long swim out to get any depth but once you get out even with the coastline(out of the cove) you will hit 45 to 50' and a sandy bottom. Tons of Abalone there and nice kelp forests with plenty of fishies hanging out.
I just went to Fort Ross Last weekend for the first time and did some Ab diving at the South end. I brought my Diveyak and we paddled around the south tip and ended up with some very nice abs, between 8 and 9 3/4", all taken from a hole @ 25 FSW (I have a home-made float line on an extra ab iron, so it helps me get back to the right spot while freediving!!)
It was nice to drive down to the parking spot and unload, but it is a bit of a walk to get back up to where you can park your car.
If you ever need a buddy for a sonoma dive, I can meet up with you in Vallejo or something?
Brian