Got this from one of my offshore co-workers. Meet Bathynomus giganteus. It latched on to one of the deepwater ROV (remotely operated underwater vehicles) we use, for example, when we are surveying downed platforms in the Gulf of Mexico. Hard to tell scale but since those controls fit an adult hand, I am guessing 12-15 inches. They live on the bottom in very deep water, but could you imagine bumping into one of those when you are diving? Of course, our coonass divers would probably radio up for his tender to put a pot of water on to boil...
They are the undersea equivalent of rolly pollies - same family as pillbugs and the woodlouse.
I also found the other two pictures on Wiki - those eyes are reflective - and the stuff of nightmares and comic book supervillans...