Van Beach - if your fine in a fishing boat then even in the rough seas you won't have a problem. Don't worry about it. Take some meds with you so you have them just in case but from my experiences if you are fine in a small boat the ship will feel very stable to you.
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