What makes cephalopods long for the stars? Do their tentacles itch for the heavens? Are there deep-sea sharks that drive them onward? Are they simply curious what hard vacuum feels like? Who can understand the mind of a creature with no skeleton?
Come to think of it, though, you'll certainly have time to figure it out. You signed on five of the silly creatures, and though the portholes occasionally distract them for hours at a time, they've taken to shipboard life quite well.