As Cruise fanatic mentioned, there is a tremendous diversity of clothes, even on formal night. Although I wore a tux on the Constellation, the two other gentlemen travelling in my party wore suit jackets, and didn't look out of place in the least. Don't worry if you don't want to be too "formal"
One couple at another table did come in wearing a flannel shirt and jeans on formal night... they didn't turn them away at the door, but they did get a lot of stares... THAT level of casual was probably unusual. Not that they seemed to mind!! LOL