Near end of dock H at Woodley Island marina, with Balanus nubilus.
Never seen such a jazzy one - looks like maybe I. urotoma, but the coloration...? See about 2/3 of the way down this page: http://tolweb.org/notes/?note_id=4182
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.
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: