It's a cloth pocket hanging device for other Guests to leave gifts and messages for other guests.
Quote from FAQ thread "So just what exactly is a “fish” and just why does it need “extending”? Well… The “fish” is a combination wall decoration/ornament and message clip that is attached to the wall outside every cabin on the Magic & Wonder. The CMs (that’s “Cast Members” for the newbies) use them to leave messages, official forms or invitations for the occupants of the cabin as needed. It’s kind of like a glorified fridge-magnet"
These post's from the FAQ thread at the top of the post's should help you figure them out more and the second one show's how to make one.
The FAQ thread has loads of basic information about cruising on
DCL along with links to threads on each subject, and although the search function isn't great, a lot of the time Google will bring up threads on the Disboards that their own search tool doesn't.
Hope you good with a needle!
Ex Techie
http://www.disboards.com/showpost.php?p=33184281&postcount=2
http://www.disboards.com/showpost.php?p=30302583&postcount=609
tamarr44 beat me too it!