Typically each cruise has at least one bridge tournament. You can ask when you get on board if there is one planned. As for other card games you may have to ask if you want to try to have one arranged. If you ask early in the cruise they may be able to put an announcement in the activity letter.
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