Open water near Napa, CA

Quick question for any locals out there... my wife and I will be in Napa, CA this coming week-end for a wedding. She is doing some open water training. I'm trying to gather some info about good places to do an open water swim while we're out there. We're aware of the Dolphin Club/South End Club at the San Francisco Aquatic Park, but that's not terribly convenient to Napa. Is there anywhere good to swim on the north side of San Pablo Bay? Thanks all. -Rick