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Which cruise line in particular.
Any really as i would not book with a company that forbids i.
There are no ships that forbid it, some are more restrictive than others ie if you can smoke in your cabin or not or smoke on your balcony (if you have one) or not, some allow you to smoke on deck one or the other sides of ship, and mostly there are certain bars/lounges that you can smoke in. it is a definite no to smoking in the restaurants and theatres now, but even the strictest of ships i have been on there is still a place to go.
Princess (Star) allows smoking in cabins, on balconies, certain areas of the cocktail lounges, casino and certain areas of deck, including outside bars. The extractors are excellent in all inside areas where smoking is allowed and those in my party who didn't smoke were never troubled. In fact, two passengers who were very 'anti smoking' were askance when someone nearby lit up, never thinking they were in the smoking area of the bar it being so very well ventilated.