I just got back from a cruise last week, and on formal night there were two groups in the Grand Dining room:
Those dressed appropriately, and those who weren't but either didn't care or didn't know the difference.
Of the 2000 passengers, apparently 50 of them think Bola ties and a vest constitues formal wear.
(cruises out of Galveston are mostly Texas/Oklahoma cowboy/redneck/NRA/AARP crowd).
I'm glad I took Paulos suggestion to wear my "undertaker drag" dark blue pinstripe suit,
cause the rest of my table was wearing tux or white dinner jacket.
Recommendation: buy your table a bottle of wine ($35-$50).
It makes you a hero to the people you're dining with all week.
Others will hopefully do the same on following nites and you'll have dinner wine all week (not included in cruise price).