You can check it, two free checked bags with Southwest! Unless I'm missing something, the "carry on" tag just means that you could carry it on if you wanted to, but it should also be fine for checking. I'm not sure about the cruise part - does the cruise line pick up your checked bags automatically at the destination airport? (Like Disney's Magical Express?) Otherwise I'm not sure that you will avoid carrying it around when you leave the airport. Hopefully the ship has a bellman to hold luggage if needed.
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