Cash is cash.

Credits are limited by all sorts of rules. There is no advantage to the consumer from a credit unless the cruise line offers a substantial premium for those willing to accept it (25% or more). The cruise line may fail and then you are an unsecured creditor who may see a few cents on the dollar a few years from now. Perhaps it will not operate what you want to where you want to go and offer the hard and soft product you want. Perhaps you will want to do something else with the cash.

Letting the cruise line keep your cash is a 0% loan. You can do better with a statement savings account!