I book luxury cruises for a living and I can tell you we get complaints galore from non-smokers who hate smoke wafting under the door into their cabin from connecting/adjoining cabins. I have experienced this in hotels also. There are numerous places to smoke onboard and you're not going to be in your cabin all the time anyway, so I don't see what the big deal is. You'd also be out on excursions at various ports where you can smoke too.
This will become industry wide, just as it did in the airline industry. European cruise lines already have non-smoking policies, by the way, in spite of the perception that all Europeans smoke. For the small minority of people who would cancel a cruise on a fabulous cruise line for this reason, the cruise line will gain more people who didn't sail previously because of the smoke they were forced to breath under the door and while out on their balcony.