The cruise lines would have to do this voluntarily, I doubt the US has the power to commandeer them. I don't think it's a particularly good idea given the character of many of the people who would be living on them. They'd be stripped to the bulkheads.
Just a thought. IF NOLA IS finally ordered to be evacuated, couldn't the refugees be flown out? I know the airports are out, but couldn't I-10 be turned into an emergency airstrip? I have read that when the interstate system was being built, it was to be built so that they could, in an emergency, be used as runways. If I-10 west of NOLA is passable, couldn't that be used? If not I-10, Take them to Baton Rogue or Lafayette. They have airports that could be used for whatever the final destinations would be. I know this sounds bizzare, but this is a bizzare situation. Please don't flame me for this.