Arthropods include the group called crustaceans, which are made up of decapods (crabs, lobsters, shrimps etc), Amphipods (usually called sandhoppers) and Isopods (woodlice and lots of aquatic beasties). Plus a few obscure groups.
The elongated shape of that thing suggests an isopod to my eye, but it could be a weird decapod. In the antarctiv there are no decapods at all, it is all isopods, and when I used to work with the British Antarctic Survey back in the 1990's I saw a few beasties that looked a bit like this thing. I'm no expert though, especially not on aquatics.