This is apparently an air bladder of Bull Kelp (Nereocystis luetkeana), but with a much shorter stipe (or stalk) than that usually associated with this species of macroalgae. Carol found this beautiful organism when we were on a walk at Half Moon Bay State Beach late this afternoon (I walked right by it because I was too busy looking at Sanderlings and Mew Gulls).
Peter Alden and Fred Heath, National Audubon Society Field Guide to California, p. 87.
A. L. Baker, An Image-Based Key: Algae (PS Protista), Cyanobacteria, and Other Aquatic Objects, Nereocystis.
Washington State University, Intertidal organisms EZ-ID Guide, Nereocystis luetkeana (Bull Kelp).