I know all about the airline travel luggage lock stuff, but I wasn't sure if the cruise lines also use TSA or something like that? I never bothered with the TSA locked for my airline travels since I carry on, but I may for this case.
I am just being paranoid about them leaving my luggage out in the hallway, or me having to do it when departing.
You need to lock it before you hand it off to the porters at the pier and again when you put it out for pickup on the last night of the cruise. In fact, Carnival even says to do so in their FAQ for luggage. If they need to inspect the inside, they will hold the luggage and contact you to come down an open it.