From the Ocean Beach Bulletin:

On Saturday morning, beachgoers were surprised to observe a tremendous concentration of what appear to be moon jellyfish densely packed along the sands of Ocean Beach. The jellyfish, packed in about a 20-foot wide swath, can be most predominantly seen between Pacheco and Lawton Streets.

You can also see more pictures of the horror over at OBB.  Personally, I'm a bit disappointed that they didn't get any shots of the beach patrol vehicles having to traverse this minefield, but there's always next time.