We just returned from the Carnival Legend and had a wonderful time. The ship is decidedly a Carnival ship, in that you can see the Joe Farcus touches all over it, but the ship itself is great. Giving something back to Joe, some of the goofiest looking chairs were also some of the most comfortable chairs!
We've done two cruises to nowhere and on both, they had a "formal" night. However, for those, formal meant no jeans or shorts, and hopefully a jacket, but no one got thrown out if he didn't have one.
If you want to see if you can get reservations in the Golden Fleece Supper Club, I would definitely take a jacket and tie as it's more "formal" than the rest of the ship.
Hope this helps!