Ever ship I have been on I can remember a grab bar being there. But the "raised toilet seat" part may be subjective. If in doubt, it would be safest to ask when you book your cabin. They have cabins with handicap accessable bathrooms. I don't know if they have various types of accessible or not (assuming you don't need one big enough for a wheelchair or something.
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