Your in a ship on an ocean. Anything is possible. The bigger ships are very stable and the waves don't normally get big enough to cause issues. I have been in 20 foot seas that don't seem to bother the ship that much. However, there can be stronger winds in which you will feel the waves. No cruise ship will ever put itself in the position to comes close to a hurricane though. I really wouldn't worry about it this far in advance.
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