You say Carnival cruises but I would check that very carefully and please come back and tell us. I don't know of Carnival participating in anything like that at all.
Generally those offers come on a smaller 1-day cruise to Nassau cruise line. I think ship is Imperial Majesty. It is all a scam - once they get your first payment they tack on a bunch or surcharges and special conditions that in the end makes a regular cruise a far better deal.