There are hair-raising stories on the Internet about how the value and appreciation potential of cruise art have been grossly misrepresented to passengers. It strikes me as an insane way to buy art -- I'm always reading about people who impulsively plunk down thousands of bucks to buy something on the spur of the moment without doing any research or comparative (objective / expert) analysis.
Too many people come ashore and discover their new purchase is worth 10% to 20% of what they were assured earlier, and there's no recourse. The cruise lines flatly refuse responsibility for the tactics of these hucksters.
I would never go near an onboard art auction.