I spoke with an AT&T rep the other day, and she said Carnival has contracts with several carriers, but not AT&T. AT&T has international texting rates, data rates, and voice rates
that are not unreasonable; however, you will be billed for a full month on the texting and data rates even if you activate them the day before your cruise and deactivate them the
day after you get home. As for the voice rate, that will be pro-rated when you deactivate it based on the number of minutes used.
You only pay for outgoing texts, as incoming texts from the U.S. are covered under whatever plan you have.
Always wise to check with your individual carrier to see what they can offer for the time period that you'll be sailing.