That depends on where the cruise will be going, or what provisions the ship has. You'll need a server that you can access. About the only phones I know of that could communicate phone to phone without a server are the Nextel Direct Connect phones.
Buy a couple of cheap Motorola or Uniden walky talkies and you can communicate as well as a phone-to-phone connection with no server needed. (IOW, you won't get from deep in the interior of the ship to deep in the interior of the ship - phone or walky talkie - unless the ship maintains wifi service - in which case you can use Skype.)