While there are many restaurants that charge an extra fee, you don't have to go to them. There is plenty of free food. At each meal you can have water, lemonade, coffee or tea. There is also milk at breakfast.
Alcohol is extra, but fairly comparable to most places on land. They will try to hustle you drinks in commerative cups which cost exxtra. If you don't want them be sure to ask for a plain glass. There are however great shows, seminars and movies which are free and probably wouldn't be available at an all inclusive.
Plus you get to wake up in different ports in the morning!