A lot of cruise lines supply luggage labels with your tickets. Royal Caribbean, Celebrity, Azamara, P&O, Cunard, Princess, and MSC all provide a personalised bag tag with your E-Tickets. This means you can print off as many as you need and attach them to your cases.
Don't worry, if you haven't received your luggage labels in advance, you can pick these up at the port and complete them there, attach them to your cases and hand them over to the porter. You should also be able to download a generic luggage label from your Customer Portal.