This odd little thing was flailing around by itself in a very small rockpool on Drake's Beach at Point Reyes National Seashore
in Northern California. We weren't sure whether it belonged in the pool, as it didn't seem to have an effective way of swimming and kept spinning upside down, but when we tried to remove it to dry rock, it curled itself up into a ball and rolled itself back into the water. We tried moving it three times.
It was about 3" long and had a shiny black and red segmented exoskeleton, many bristly little striped legs, and was rounded at both ends, with tiny antennae on a little cartoon face.
We didn't find any of its friends nearby.