When I went out there on a similar day, I came across a 182 doing airwork at low altitude near Bolinas. And that was it. This was on a beautiful Spring Saturday.
Arriving in Petaluma, there was a local swarm of ultralights. But no traffic around the bay, aside from the KSFO conga line.
For several other bay tours, I've been shocked at just how little GA traffic there was.
Most recently, I needed currency in the Archer, and some practice with the autopilot, so I decided to try out some Class C ops at KOAK and then move on to Sonoma Skypark. NO traffic. Nada. Not even at fricken' Oakland (even onto 29!). Nice, somewhat warm late-summer Sunday afternoon. NorCal cleared me into Class B on departure, which I hadn't asked for (nor was it truly necessary).