If you have your meals in the dining room (or Lido buffet), you do not have to pay anything extra - unless you want a soft drink (Coke, etc.), or wine or mixed drink. All other beverages like coffee or iced tea, no charge. (Carnival does not serve Pepsi products...Coke products only.)
Any ship that has the "specialty dining", such as supper clubs and sushi bars, DO charge extra for those venues. However, the dining room and Lido buffet, no extra charges. All you can possibly eat - no charge.
Also, if you order anything from Room Service, there is no charge - except for alcoholic beverages or Cokes. All the food, juices, tea, coffee, etc. - no charge. (Just tip the room service delivery person $1 or $2.)
In case you are not aware of this - There will be six soft drinks (Cokes/Sprite) in your cabin when you arrive, along with a liter bottle of water. There IS a charge if you drink these. Just wanted you to know they are NOT free. The Cokes are $2.50 each and the water is about $3.75. If you drink them, your cabin steward will put them on your S&S account.