Smurf - Do check in advance, but as a rule, there are no dress restrictions for the buffet section on a modern cruise liner.
There are usually a couple of 'formal nights' in a week, where a tuxedo for men and an evening outfit for ladies is required if you are eating in one of the formal dining rooms, but otherwise, it's usually smart casual in the dining rooms, and jackets are not required.
During the day, you can wear what you like everywhere on board.