If the tour doesn't include the tickets, what are you paying for? I hope its not a lot of money because getting to Pisa and then to the tower is fairly easy by taxi/train. You can buy tickets at the tower. The length of the lines to climb it very by time of day and season but I would expect there to be a line. If I were you I would wait until I got there to buy tickets. You may take a look and want to back out of climbing it.
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, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013