What does one wear on a cruise?

[Ohwhatfuckeryitistoride]

DH is sniffing around cruise adverts(which means we'll be going on one) but I've no clue what to wear?(my concept of cruise wear varies between Agatha Christie and Abigails Party in style) It will be adults only and probably older passengers. Don't know where yet or when yet. I've seen some say formal-is that long dresses and suits? Do I need to invest in kaftans and sparkly frocks?

And yet, you still see people attempting to gain entry to eateries while dressed in such a way.

Though not so much when I was in Norway last year.