Carnival cabins have a mini fridge stocked with little booze bottles, wine, and soft drinks, which you will be charged for if you used them. You can ask to have the fridge emptied to accomodate your own stuff.
We usually bring some wine, which we uncork ourselves in the room, and sometimes some rum for hubby, which he has with the Coke that the room steward keeps stocked in the fridge for us.
Freebies in the Carnival Lido buffet include water, ice, fruit juice at breakfast, iced tea (weak and unsweetened) and lemonade. Also, regular coffee and tea. All other drinks must be bought at the bars around the ship.
If you drink a lot of soft drinks, purchasing a "Coke Card" is a good idea.
For one weekly fee (around $30 I think now) you can have free soft drinks at any bar (no dining room or rooms service).
Hope that helps...