You can bring cans of non-alcoholic beverages -- nothing in plastic. I believe the can size is limited to 12 oz.
Carnival Policy: On embarkation day, a small quantity of non-alcoholic beverages (i.e., sparkling water, sodas, energy drinks, juice, and milk), packaged in cans or cartons, may be brought on board and must be in the guest's carry-on luggage. A small quantity is considered a maximum of 12 sealed, unopened cans/cartons of 12-ounces/354-ml each or less, per person