Carrie if you look on the picture that Lisa/Disneylover5 posted, you will see the 'fish extender' hanging from what looks like a silver fish coat hanger. It juts out from the 45ddegree wall (which is where bathroom part #1) is thus the angled wall) anyway, that's basically what it is - a coat hanger that hangs outside your cabin door.
Anyway,
DCL uses to drop off notes for the guests and this is where the idea of a 'mail delivery' came into play. So in the picture - since its on a rope or dowel, you attach the rope with a zip tie to the 'coat hanger' silver fish ornament and that is I believe how the name Fish Extender came into play. Many use it to drop off litte gifts which make the cruise lots of fun. Hope this helps answer some of your questions, now where are we?