The creature above is isopod Ligia, most likely the boat roach, L. exotica. The extended urosomes (rear projections) and the antennae make it possible that it's something else. It's less than one inch long and lives on sea walls and rocks along the shore.
A well-known marine scientist says, "Boat roaches are used to teach students how to derive population estimates. By drying their backs one can apply a dab of nail polish. Then they are let go in the same area they were collected in. Come back a day or so later and recapture them. The ratio of marked to unmarked critters allows an estimate of the total population."
(It has been suggested to the photographer that she capture a shot of its underside and side view. HA !)
(Web viewers are welcome to send in their photos of this creature.)