Another option to consider if you are looking to save airfare... and this completely depends on where you live and which airline you choose to fly on... but my sister found it much less expensive to fly into Dallas (DFW airport) and drive to Galveston (one way car rental and about a 5 hour trip) then to fly into Houston.
BUT like I said, it is highly dependant on your air travel costs to Houston and which airlines service your nearby airport versus Houston. IF you are already looking at renting a car to get to Galveston from the Houtson airport, factor the cost of the rental in Dallas into the trip travel cost total as well.... Just a thought.
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