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You can smoke on deck, on your balcony (not necessarily in the cabin) and in the designated smoking areas on SOME larger ships e.g. one of the bars. This ruling is regularly changing at the moment so check the small print. Fire is the biggest threat to life on any vessel. NEVER smoke in bed or throw lighted materials over the side, they can be blown back and start a fire.
The situation is changing almost weekly, very few ships now allow smoking in cabins in fact certain cruise lines will charge you $250 to "clean" a cabin if you smoke in it (and they are most fastidious about this).
To get a more accurate answer you will have to be far more specific about either what your requirements are or which ship you are thinking of using. ... Neil.