Cruise Ships do have security guards who do carry weapons in the event they need them.
I've also heard that some cruise ships do carry additional weapons on board beneath the ship in case they need the weapons because of an attack.
But it's rare that the cruise ship would need to use the weapons as an attack on a cruise ship is really rare.
You're more likely to get into a cruise ship accident and it sink than you would be likely to get attacked by pirates or anyone else.
Cruise Ships also have a jail below the deck of the cruise ship where they can hold someone if they need too until they reach land.
So if you're caught stealing or harming someone then you can be arrested by the guards on board of the cruise ship and then be held in the cruise ship jail until you get to land.
Then they will transfer the prisoner to the local police who will then take you to their jail.