The specimen very likely appears to be member of the Saccocirridae Family of polychaetes of the Subclass Polychaeta Incertae sedis (temporary). May be immature Saccocirrus sp., but can't say for certain. However, the species does appear akin to a member of Saccocirridae.
Your specimen is a dipteran larva. It looks very much like Ephydra, a genus of shoreflies/brine flies (Ephydridae). If you have access to the old (and somewhat out of date) "Aquatic Insects of California", check out the illustraion on p, 472. There are other families of aquatic larvae that somewhat resemble your photo- Empididae & Syrphidae. However, from what I can see, shoreflies are your best match.