Well, since I work for Motorola, I'd suggest the Motorola walkie talkies
But any set that you get, make sure you can select different channels. Numerous people on ship will have them and if you can't change channels you end up listening to everyone else. The Motorola radios have lots of channels and sub channels, so you can usually find on that is free.
The postie notes is a great idea. I'll have to add it to the cruise list. We always ended up using whatever scrap of paper we could find.
As far as cell phones, don't worry about them... Once you are about 10 miles from the US (or mexico) they are pretty much useless as there are no cell towers floating in the ocean
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013