Arash,
I did a Google search for Osprey in the SF Bay area and it was noted that they are occasional visitors to the Bay are. Someone out there might know of a lake they frequent. The only place I see osprey every time I'm there is at McKerricher State Park north of Fort Bragg. There is one freshwater lake in the park.It has trout in it that the Osprey just love. Sometimes you see River Otters too.
They keep a running count of all migrating Hawks and other raptors over "Hawk Hill". Just scroll down and you will see the record of each day which can help you determine the frequency of the osprey that fly over the Golden Gate.