Given the reddish-orange color of the bill, the thinness of the bill, the shaggy black crest, and the location in summer, my thought is that this is an adult Elegant Tern (Thalasseus elegans).
I'm sorry to hear that this was the only tern that you saw during your trip! I've only visited California a few times, so I'm not familiar with how common each species of tern is, but I know that Elegant Terns nest in colonies along the southern coast during the summer. I don't know what would have caused its death and any hypothesis would just be wild speculation on my part.
