I agree with Sonny... Its very rare to experience a cruise with seas that bad. We have only had 1 cruise out of all of ours that had what I would consider to be really bad seas. So you already have your odd one out of the way! I would agree that leaving from a southern port will lessen your chances of a repeat. To even further reduce that chance, try not to sail during Hurricane season as anything can happen then. They can normally sail around the big storms or pick alternate destinations to avoid high seas when sailing from Florida, so you have better odds (even in Hurricane season) to be in good weather.
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