Suzy- Well, yes its ture that if you can stand on the aft of the ship the smoke will travel directly off the ship... BUT where you can do this is going to change from ship type to ship type. Most of the newer CCL ships have balcony cabins on most of the decks except for the dinning room deck. So the Lido deck and higher are probably the only places you can get to the aft of the ship. On the Fantasy class ships there are areas where you can do what you describe but its been a long time since being on one of those, so I can't say for sure which decks those are.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013