I believe the "formal optional" is only for 7 day or less cruises. At 8 days or more, there will be "formal" nights that are supposedly not optional. Last year on two cruises, Moon and Dawn, for formal nights I wore black slacks, a Black sportcoat and a tie and didn't have a problem anywhere on the ship. I did see people going into Atlantide without a sports coat and they were given extra sport coats to wear. Put the coat on your seat back if you are too warm.