Cool tail shape. About 1" long. ID as per Doug Mason's 2014 photo: https://www.flickr.com/photos/39365853@N07/15669466241 and this guide: http://tolweb.org/notes/?note_id=4182 (scroll about 3/4 of the way down).
San Diego County, California, US
I found this useful key online
OLYMPUS DIGITAL CAMERA
in eelgrass bead near smith river mouth.
This died on my friend's towel at the beach
Eelgrass isopods mating.
Idotea is a genus of isopod crustaceans, mostly from cold temperate waters. The taxonomy of the genus is still in doubt, and many of the currently recognised species may be taxonomic synonyms, and others may be moved to different genera. The genus Idotea includes the following species: