Not an octopus this time. This is the underwater giant woodlouse, or roly-poly or pillbug. It looks too big to roll or pill like the little ones. Generally I don't mind crustaceans - it's insects I don't like, but this is quite a disconcerting one. Luckily it lives at the bottom of very deep water, so our paths are unlikely to cross.
Apparently they eat dead whales and squid. (The ones in my garden eat my compost heap.) They're isopods (their legs are all similar), and they have a lung structure that means they can live on land (as long as they can keep themselves damp) or underwater. The underwater ones are called Bathynomus giganteus and there's a photo of an even bigger one here.
A relative is the disgusting Cymothoa exigua, an underwater louse that eats the tongue of fish, and then, in a sort of fair-dos exchange, lives in its mouth and performs the actions of the missing tongue (and grabbing the occasional tidbit). When I first saw this, it made me wonder about other possible organ-replacing parasites as a science fiction plot idea. If you could persuade something with good pumping power to replace a failing human heart in exchange for the free sugar and nutrients in the blood, for instance. What type of society would those parasites build? (Parasites are often solitary and sometimes hermaphrodite, not even needing others of the same species to reproduce. But that wouldn't be as good in a story as something that needed to find others of its kind to mate with.)
Picture here at Animal Planet.