I'm sure the ladies will have more advice for you, but just from a man's observations, any of your choices would be just fine--no, it dosen't have to be an evening gown.
What is the ship like? That's a pretty broad question--I'll just say it's large, beautiful, even elegant; many diverse venues for dining and entertainment. It's sparkling clean and very well maintained and the staff is very efficient, courteous and friendly.
Have a wonderful cruise. If I can be of any more help, please ask.