There are printers available on the ships, but check the airline policy on the time when online check-in becomes available. Sometimes, the cruise lines close the computer centers at the end of the cruise to faciliatate the disembarkation process. I think Southwest now allows online check-in 24 hours prior to your flight, but check the rules before you leave on your cruise.
Otherwise, you can use the kiosk check-in option at the airport. Just be sure that your flights leave no earlier than noon and get to the airport early!