When we got closer, the vulture had flown off, so we went over to look at what had attracted the vulture's attention:
At first I thought it was an electric ray, but there were several characters that didn't seem right. After doing some more research, I believe this is a Thornback Guitarfish (Platyrhinoidis triseriata) — the first I've seen.
It's especially important to note that the caudal fin (at the tip of the tail) is relatively small; the two dorsal fins (on the right side of the photo) are about equal in size; and there are three rows of prominent spines running down its back and along the tail (sorry I don't have better pictures of those!).
Thornback Guitarfish prefer shallow water and sandy or muddy bottoms. They are known to occur as far north as Tomales Bay. Have you seen them in Bodega Bay, or anywhere further north?
P.S. To compare with a Pacific Electric Ray, review the post from 5 February 2014.