An estimated 3,000 diving ducks (primarily Greater Scaup) and other assorted waterfowl/Grebes are in several big rafts at the county line between San Francisco and San Mateo.
This is an annual mid winter occurrence but definitely worth taking in before these flocks move off or disperse as well as an opportunity to find less common diving ducks in the mix.
Good vantage points for San Francisco birders are from Candlestick State Recreation Area main parking lot, candlestick point and Last Port. A bit tougher for San Mateo, with no publicly accessible sites I'm aware of besides walking along the shoreline at low tide from Candlestick SRA "Last Port" unit.
Most of the birds are however technically south of the county line in San Mateo hence the post to both lists.
Scope absolutely essential to observe/count/sort through all but the closest rafts.