No charter boats on the North Coast that I know of, I just moved from that area a week ago. Nearest dive shop is in Santa Rosa to the east about 2 hours, or Ft. Bragg to the North about an hour from Sea Ranch. You can get air fills at a hardware store in Gualala, just a few minutes north. They may have some gear, but probably not rentals.
Lots of great diving, theres shore acsess near Sea Ranch, usually involves a lot of hiking though. About 30 minutes south is Gerstle Cove, very good diving, easy access but rocky, watch your step. A few minutes south of there is Stillwater Cove, easier access, good diving and a sand beach.
Northern CA diving is great if the conditions are good. This area is very remote and services for a diver are limited. The abalone diving, which is open in June, is great if you like freediving. Have fun.