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From the Carnival Website : Guests are prohibited from bringing alcoholic beverages onboard. However, guests 21 years of age and older only may bring one bottle (750ml) of wine or champagne, per person, on board only during embarkation at the beginning of the cruise. A $10 corkage fee per bottle will be charged should you wish to consume this wine in the main dining room; $14 corkage fee per bottle in the Supper Club. Guests may bring a small quantity of non-alcoholic beverages. All prohibited alcohol, additional quantities of wine/champagne or excessive quantities of non-alcoholic beverages will be confiscated and discarded without compensation.
All the cruise lines say this but we have seen many get away with it as the people at the gangway don't really care it is only if there are other officers around.
I agree with Pearson's comment on initial boarding, 'blind eye' 'don't upset the guests' 'keep them moving' all come to mind. But re boarding at ports of call are a different matter & most lines are vigilant & 'confiscate' liquor till the the last night of cruise, so be warned.
Depends on the length of your cruise 1 to 5 days 1 formal night and 6 to 17 2 formal nights.