I know many cruisers prefer private tours, but I still do mostly ship sponsored tours. In 6/09 on the Summit Classical Med we did a 10 hr day with a walking tour and lunch in Florence and two hrs in Pisa. It was plenty for my friend and me. Too much for my cousin and her bad back. She likened the walking tour of Florence to the Bataan Death March
and changed her 10 hr tour of Rome to 5 hrs.
On any cruise you have to understand that you will not get a really comprehensive tour of any port or surrounding area. Pisa is not a large area and easy to do in a short amount of time. I don't want to exhaust myself doing too much and not enjoy other aspects of the cruise.
In Athens, my friend and I ended up doing it on our own because the tour guides were on strike. We probably wouldn't have survived the 10 hr tour in 95 degree heat. By using the ship shuttle, walking to the Acropolis and riding the double-decker bus afterward it took 5 hrs and we were barely breathing by the time we got back to the ship. We had filled up our water bottles at the Acropolis restroom but it didn't last long. In deference to my friend's age (77) I let her have the shower first while I drank water like it was going out of style. We found some lunch then came back to the cool room and collapsed.
I love cruising the world but don't expect to see much more than the highlights.