From the Carnival web site.
What should I pack?
Casual attire is the order of the day, both on board and ashore. For one or two nights, a dark suit or formal attire is suggested. The dress code for other evenings ranges from sport coat and tie to casual resort wear. For Alaska and Canada cruises, sweaters, lightweight jackets and raincoats are suggested.
Note: Shorts and T-shirts are not permitted in the dining room area during dinner.