This is the subject of many (some heated) discussion on a cruise forum I go to. Most cruise lines have turned formal night into a suggested dress and not a required dress. I am going on the Oasis of the Seas in September and will be wearing a PC, the suggestion is Tuxedo or dark suit. In highland dress terms I suppose this would be PC or Argyle. I myself am just that stubborn and refuse to let the masses take away all of the formality in dressing for dinner.