Good question and despite having spend nearly a year (total)in Italy I am not sure of the answer. The train ticket you buy to Florence does have a time limit (I thought it used to be about 2 hours) which might not be long enough to stop in Pisa, get all the way over to the Tower and later back to the train station (there is actually another smaller station closer to the tower which is not used by most of the trains). You might have a port lecturer on your ship that will no for sure, but on RCI the port lecturers usually want to only sell their own excursions (they usually work for a franchiser which handles all the tours). Personally, we love Florence so much it is hard to recommend that any first-time visitor cut a visit to Florence short to visit Pisa. Even a full port day does not begin to do Florence justice (we never loved that city until we spent a few nights there at a local hotel).