DH and I are currently om Sunshine, which is scheduled to arrive in Grand Turk tomorrow. Tonight when we returned to our carbon, there was a note next to our door that said that Carnival did not have proof of our vaccination and if we wanted to go ashore in Grand Turk, we needed to visit Guest Services and provide it.
This surprised us since we are fully vaccinated and boosted, have taken many Carnival cruises since the Covid restart, and have provided proof of vaccination whenever asked. We went to guest services and showed them our vax cards, and we are cleared now.
Long story short, take your vax cards with you on your cruise. You don't know when/if you are going to need them.