dbug.. check out a one way rental from AVIS. We are doing that and it costs $61 (for a mini van)(plus your gas) which is less than the cost of one transfer. So if you have mutliple people traveling, that may work for you. AVIS has a shuttle to the port.
If you want to have it two days, double that price. But check with your Hotel. If they offer a shuttle to the port, then maybe the rental car shuttle can drop you at your hotel???
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