Just found this thread, don't know if it still matters, but…you can take public transit from the airport to SF, and get within spitting distance of the Embarcadero. You want a Samtrans bus (if you arrive early enough) 292 or (later) 397.

I've never taken either of these into the city (although I used to take the 292 to the airport within San Mateo County) so I can't speak to safety and whatnot. These are commuter buses for airport/airline employees, so you'll be riding with a lot of them at the start, but I don't know what the clientele is like along the whole route.

They are, however, cheap! More info here:
Samtrans 292
Samtrans 397

Oh, yeah--Samtrans, BTW, is San Mateo County transit, San Mateo County is where SFO (the airport) is actually located.