The trip from the airport to Galveston is at least 40 miles. I don't think a taxi is your best bet there. Maybe a Limo or similar form of transport. Also check with your hotel, maybe they can arrange transport to their hotel.
Sorry I can't be much more help than that, I normally drive my own vechile to the hotels/ships
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