If you want to find information on this, you can use the search feature of this forum. this question comes up way too often and we are not going to start another one on the topic.
It is against the cruise lines' rules (unless you are talking about wine). Even that can be found in many topics if you use the search feature. Sorry to be so heavy handed but this topic never turns into a good discussion.
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
NCL Dream October 2006, RCCL Mariner OTS June 2007