You apply them to your stateroom account. They accept them like cash to pay down your bill. Don't believe they can be used to pay the cruise, unless you go directly through DCL as a TA or the like won't accept them as payment. I would call DCL directly for that.
You're better off not applying them toward the cost of your cruise and just using your Visa instead. The reasoning is that the more you spend on your Visa, the more rewards $$$ you will receive. You can then use those toward your shipboard account.
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