I'm not sure but they are central time zone and I don't think they do the daylight savings time thing like in the US (Like Arizona doesn't). So during DLST they will be two hours, but during CST they will only be one.
Having said all that, when you are cruising there, they will tell you to follow "ship time" which will be Florida time. They don't typcially change time on the ship unless you are going through multiple time zones. If they do, they will announce it the day before in the daily activity paper.
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