There is no reason to spend an extra penny for food on a
DCL cruise unless you choose to. All meals in the main dining rooms, the fast food outlets on deck 9, and the "snacks" outside the nightclubs in the evenings and late night are included as are the pirate and dessert buffets. Sodas are free in the dining rooms and the beverage station on deck 9. Juices, tea, coffee, etc. are free.
What you can CHOOSE to spend money on:
Palo (adult only) restaurant $15 per person for brunch or dinner, $5 per person for high tea
Alcoholic beverages, smoothies, specialty coffees (at Cove Cafe)
Popcorn, candy outside of theatres (you don't need to do this; you can get snacks from deck 9 and take them with you!) or from room service
Sodas served in cans from any bar or room service
bottled water (again, not needed...ship's water is good!)
You are allowed to bring your own sodas, alcohol, etc. on board with you. If you take a bottle of wine into the dining room you will be charged a "corking fee," but your room host will supply a corkscrew and glasses and you can open it and pour it in your room at no charge.