Cruise ships have their own private security guards which do have arrest powers but they are not the regular police that you would call 911 and have show up at your home or place.
The cops that are on cruise ships are not really cops but instead private security guards with arrest powers where they can legally hold you in the jail of the cruise ship if you commit a serious enough crime.
Cruise ships do have a jail on them to hold passengers that commit crimes.
The person who is jailed on the cruise ship will be held their until they reach land again and can transfer you over to the police.
The cruise ship has guards with arrest powers on them so that they can stop crimes on the cruise ship.
So if someone is thinking about going on a cruise ship and committing a crime and thinks they can get away with it because they are at sea away from police then they should think again.
Guards are on board cruise ships for passengers safety and they can and will arrest you and take you to the bottom of the cruise ship which is unpleasant and hold you there for the entire cruise.