The steel superstructures on ships can inhibit their range and the three mile FRS type walkie talkies will not communicate in all areas of the ships, even 60,000 GRT ships.
I use a GMRS radio and have no problem but still have some "dead spots" on large ships.
If you are in the lower decks (inside cabin) and someone is on an upper deck you will probably not get good reception, with a FRS type walkie talkie.
If you are going to use them I suggest that you use walkie talkies that are at least FRS or GMRS(Longer Range) and with multiple side bands. This way you can have at least a "semi private" conversation and so can others. It is also a good idea to use them with discretion and not in the dining room or other public places unless it is VERY important.
Some places require licensing of GMRS radios but they are sold over the counter in almost every electronics store (i.e. Frye's, Best Buy) in the U.S. and license applications are, basically, automatically granted. To be used on a cruise ship there is little or no problem. FRS is no problem in other countries, GMRS there may be a possible problem but little chance, other than Hong Kong or Brasil.
If you use a UHF radio then you may have an issue.
__________________ Cruisemates Community Leader/Moderator
"There is a great difference between being well traveled and just having been to many places." ~Me