Linktrained, not a pilot, but I've landed at SFO many times (live there). Seawall is probably not really descriptive, as I do not believe it is a wall that rises above the plane of the runway. Rather, it is a sloped edge down to the water with large rocks that essentially prevents erosion of the area due to wave motion in the bay. The bay, being sheltered, does not really ever get high waves, but it can get pretty choppy when the wind picks up and the tide is in.
You can see it pretty clearly in this Google Maps image:
http://goo.gl/maps/j26Rq