Confidence: It might be this.
Notes: "Most of body is taken up by a thorax composed on seven sections (somites). The body ends in a short abdomen (two somites) and a distinct tail-piece (the pleotelson). The pleotelson in adults is characteristic, with slightly convex sides and a posterior end that is rounded in juveniles but bears two side processes in the adult giving a notched or emarginate appearance."
-From MarLIN. Possible ID based on ther notched pleotelson and 7 thoracic segments? 10mm long, found clinging to a piece of seaweed dragged to the beach on fishing line from beyond the breakers- well below intertidal.