Wildeone you nailed it.
I believe mine is a Serolina delaria (Sand skater)
Sand skaters use their discoid shape and camouflage to swim and walk about invisibly on the surface of sand in marine environments. They feed on the organic matter found in the sand.
I believe yours is a Cymodoce coronata (Marine pill bug)
Marine pill bugs are scavengers and browsers, feeding on living and rotting algae and other debris on the sea floor.
http://www.museum.vic.gov.au/crust/isopbiol.html