Cousins, my sister lives in Palm Beach, FL and her "not so better half" has a boat moored in the causeway. He wears his khaki shorts, boat shoes and logo polos everywhere he goes. The polos have Palm Beach Country Club embroidered on them. Doesn't matter the event or affair, he's in the same garb time after time.
They will see when they are at the formal night how dressed up cruisers can get. It's a nice chance to take photos. I would rather be over dressed than under dressed at any event.