A lot of cruise lines supply luggage labels with your tickets. Royal Caribbean, Celebrity, Azamara, P&O Cruises, 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. You can access your travel documents once they have been released by logging into My Booking.
Don't worry, if you haven't received your luggage labels in advance, you can pick them up at the port, complete them there, attach them to your cases, and hand them over to the porter.