Coffee, tea, juice, and water are available at meal times for free.
Soft drinks, beer, wine, bottled water ect. must be purchased at all times.
The policy is that it is not allowed to bring any type of drinks aboard but is not consistently enforced. If they see it they will probably take it and return it at the end of the cruise.
I was told by RCCL that you could bring your own water and sodas, within reason. They said you could bring alcohol, but that it would be kept by the ship until you wanted to drink it, and it had to be in the dining room where you drank it. That is all I know.