Many years ago I went on cruise to Alaska with family. I had a 2 meter VHF hand held in my carry on bag. When boarding, the bag was searched and the radio was temporarily confiscated. I got it back a few hours later, after it was likely checked by the radio officer. Not a problem, didn't offend me at all, in fact I was glad they were being proactive and doing their job. So, be prepared, although with a basic FRS/GMRS hand held, you probably won't have an issue.
Ultimately it is up to the ships master as to what you can use on board.
Since most large cruise ships are registered outside the USA, the country of registry rules would -technically- apply when in international waters, but this is rarely reality.
The country you are in would be able to dictate the rules, so be careful and pay attention to where you are and what rules apply. Doesn't matter if you are from the U.S.A., in another country their rules apply and you must abide by them. It's -always- best to ask.
The fact that some tourists may not run into issues in a foreign country does not mean that it's legal or rules are not enforced. It just means they maybe were lucky and didn't get caught.
Important thing to remember is that different countries have different frequency assignments than the USA. What might be GMRS in the USA could very easily be public safety in another. While you may not hear any traffic, it doesn't mean you are not operating on a repeater input frequency and causing major interference. Don't assume that because you do not hear anything on a frequency that it's free to use.
On board the ship, you'll find that the higher frequencies tend to work better than lower frequencies. I've heard people have had good results from a GMRS/FRS radio on board. I've also heard that the Motorola DTR 900MHz radios work well, also.