Not sure what info you wanted about Fish Extenders but I found this on another post and thought it might be beneficial for you...let me know what other questions you still have!
~~~~~~~~~~
Fish Extenders are handled on your Cruise Meet thread. If your group decides they want to have a FE group. Not all cruises do. Especially the short Dream cruises. Most of the time it's because they have a larger number of first time cruisers who feel they don't know how to handle it.
A little primer on FEs:
WHAT IS A FISH EXTENDER?
Outside your room onboard is a message holder, in most cases it's shaped like a fish:
Once a cruise meet thread is up an running, someone in the group will (should?) step up and say "I'll maintain the Fish Extender exchange list". That person is the center point for all those who want to participate in the exchange. The list can be as simple as DISnames/room number/number of guests or as involved as full names/room number/ages of guests/like & dislikes/allergies/favorite characters/whatever is deemed necessary to know to put together the gifts.
Then each room on the FE list (note this will not be everyone on the Cruise Meet thread for your specific cruise), makes a Fish Extender holder. This is basically an organizer-type hanging bag with separate pockets for each person in the room:
They also make up gifts (either individual gifts or room gifts - whatever is decided by your group) to be delivered during the cruise.
One note about the gifts, some groups set up specific delivery dates, and specify "each person gets a gift" or "only one gift per room" or (even) maximum/minimum price ranges for gifts. Other groups just say "whatever", so people can choose what/how many gifts to give. Some give individual gifts, others only one gift per room, while others might do one for the adults and separate gifts for kids. Or only gift the kids.
HOW IS A FE GROUP ORGANIZED?
Setting up FE group
There's really no "rules" as to how a FE program runs. Basically someone maintains the list of those who wish to participate. The group can decide on what information is necessary to produce FE gifts. A basic list may only have DISname/room number/number of people in the room. A more involved list could be first & last names/ages/room number/favorite character/favorite color/allergies/whatever is deemed necessary to know. Also a decision can be made as to size of FE. Some groups break out the list(s) at 10-15 rooms max, so that there are multiple lists (and some people will participate in more than one list). Other groups just make one big list (upwards of 20-30 rooms).
At some point the list will "close". Typically this happens 30-45 days out from the cruise, so that everyone participating has time to organize/prepare their gifts. Shortly before this time (about 2-3 weeks prior) the organizer will either post the list (usually via a conversation is best, as that's more private), or directly on the Cruise Meet thread, to confirm everyone on it is still participating. It’s important to make a statement about "anyone not confirming by XX date will be dropped" at this time.
Once the list closes, the organizer will post the final list. Also, a decision must be made as to whether to allow late additions. Sometimes there is a "pixie dust" list. That is, basically, a latecomers list that gets posted about a week out from the cruise that others may or may not add to their "final" list. Be sure the "pixie dust" list participants realize they may or may not get gifts from others.
The other "biggie" is no other social media sites are allowed to be linked or search directions posted as to how find that other site here on DISboards.
Now, armed with this information, you can head over to your specific Cruise Meet thread and see if anyone's interested in setting one up.
WELCOME DISNEY FANTASY WESTERN CARIBBEAN CRUISERS! 
