I've never used them on cruise ships but I am somewhat of a radio afficianado (spelling?) Look for family band, that will save you $20-$30 rich off the bat, these include 14 channels approved by the fcc for general use, they are very common so expect some interference with other people using them, the best way to counter this is to find some that use quiet codes basically you switch to the quiet code and you don't hear everyone else, the only downside is everyone one on that channel can hear you. (i.e. if you are on channel 7 and so is someone else you will here them and vise versa, if you are on channel 7 quiet code 14 you wont here them but they can still hear you) there isn't a huge variety in features ifyou just need to communicate. Motorola makes the best in my experiance. I have about 20 overall half of which are motorolla's the rest are relative crap. Ebay is a good place to look but you probably won't spend over $20/radio or under for that matter.
Hope this helps