If your plan includes "Nationwide" service that includes Hawaii you will be fine. Just watch out when you are at sea. I'm not positive that the ship turns off their cell service when they are traveling between islands. There are areas between the islands that do not have cell service and the ship's system could switch on at that point.
I would set your phone to "No Roaming" or "Disable Roaming" to assure you do not pick up the ship's cell service while you are between islands.
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Upcoming Cruise(s) Carnival Liberty: May 31, 2014. The Anniversary Getaway.
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