If you are talking about the Monster Rogue wave of the type in the make believe "Posieden Adventure" movie, then yes VERY rare if every.
There are such things as rouge waves, but they are normally not big enough to capsize a cruise ship. Even the normal ones are a rare occurance. I do not know if the modern radars can detect them our not. I would imagine if they are large enough to worry about, they could.
I really would not be too concerned about them. You have greater chance of harm occuring from stepping into your shower.
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, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013