All of the Las Vegas Casinos have joined the band wagon for offering "free cruises" on Princess, Carnival and Norwegian. You must pay your own port fees and gratuities. So as an example, a 7 day cruise to Mexico with 3 ports would cost you $110 for fees & roughly $100 for gratuities per person for an inside room. (Obviously you would have an expense getting to and from the port.) There is a nominal up-charge fee to upgrade your cabin to a balcony or a suite.
So what is the catch? When the schedules for free cruises are released, they are booked extremely fast! And obviously, it is dependent upon rolling over the money you gamble with over a period of time.
What about free offers from the cruiseline you are sailing on? Based on your play, the line may offer you a cruise for X amount of days plus X amount of cash.
What is the catch? You usually have a very short window (6 months) to book and sail on the offer.
Bottom Line: The money spent gambling (and hopefully winning) needs to offset the cost or your "free" cruise can cost you a fortune. That said, if you gamble anyway, you might as well choose places that reward your play with more than a free bufffet.