ive been on cruises with no shows. its very rare that it happens but anything can happen to cause a show to get canceled: performer gets sick with no replacement, technical issues with the stage, rough weather makes it not safe to do the show, etc.
depending on whos onboard the ship and the cruise/entertainment director they can sometimes swamp out a show at last minute. if a playlist production show was scheduled and all of a sudden they cant do that show, they either have the dinning room magician do a show or have one of the fly on entertainer do a show