Its doable. But it will be tight. Make sure that you let them know you need early disembarkation because you have an early flight when you check in. That way you get in the first to be off list. Then yes, the taxi is the best plan. Its about a 20 min drive to the airport, but you don't want to have to wait for the bus to fill up.
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