Do I Absolutely need to link a credit card?

Hi everyone i leave for a cruise on June 17th & I remember seeing someone say instead of linking a card for your purchases, you can bring cash & load it up on a card & use that to pay. I’m on the Royal Caribbean app & saw the “check in” feature on there but i cannot move forward without adding a card. Also, my partner is planning to use their Birth Certificate & ID as their documents instead of a passport & that isn’t an option on the app either. So My question is Do i need to check in from the app or will I be more than okay to show up on cruise day & check in person? Also, if anyone can clarify the usage of a credit card. I’m going on Ovation of the Seas & would like some of my anxiousness to relieve.