Once you get on board there will be clearly marked areas where you can and can not smoke. it's probably best to enquire once you get onboard.
On thomson celebration in sept 07 smoking permited in one half of one bar and on open deck all other bars and theatre non smokin.
On other Thomson ships you can always smoke in at least ome section of one of the bar/club and on the open decks, you can smoke in your cabin but it is strongly discorage.
One side of the ship is allocated for smokers no bars allow smoking on the destiny we were on it August last year, they do try to discourage you from smoking in the cabins.