I agree, Nigel.

We go on quite a lot of cruises as our holiday, and many have formal nights. It is always an option for those who don't want to dress up to eat in other restaurants, but the main ones are usually 'formal only' on those occasions. There are also plenty of very good cruise lines who have no formal evenings at all. So I don't see this as them telling you to dress up, but offering that as an option which you agree to follow, or to eat in one of the other restaurants.

You do get the occasional row at the door where someone turns up, demanding to be let in, dressed in casual clothes, but insisting they paid for their holiday and have a right to go where they want dressed how they want.

That everyone else has also paid and opted to go for a formal evening does not cross their mind.

So Nigel is right. They do not have the right to tell you how to dress. But on behalf of everyone else, they do have the right to refuse service.