We were just there in June. They allow long shorts (knees needed to be covered) for guys and shoulders needed to be covered for both men and women. So no tank tops. Skirts are okay as long as they cover the knees. yes, jeans are ok.
The same can be said for most of the cathedrals you would visit in Italy. And also for the Mosques in Turkey...
So capri shorts and a short sleeved t-shirt for women and long shorts and t-shirt for men is the minimum if you expect to be let in.
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, Celebrity Silhouette 2013