Protecting your (floating) property and passengers and crew from damage, boarding or attack is understandable, but an unprovoked assault from the decks of a cruise ship on to an innocent sightseer is not. Even HM ships' armed guards will wave you off quite politely and they've got a real reason to be alert. If cruise ships are "sensitive" now then Carnival should spring for a small boat patrol to warn off sightseeers when in and approaching harbour. (More work for the lads heh heh). They can certainly afford it.
There were a bunch of "security guards" on board B&Q off Cowes during Cowes Week last year armed with Super Soakers (gay boys) and using them on anyone getting close (which was more than a few as it was recent news then). Even doing a tour around the boat for the benefit of your paying customers solicited a bombardment if you were within range. Just made you want to board the damned thing and twat 'em good and proper like.