My family of 4 have reserved 8 seats for a tour of Rome with CBM. Anyone interested in going with us? Our cruise ship Disney Dream will be in port July 13. The all day tour cost $135 (pick up and drop off at Civitavecchia. Dorsey
You may want to see if your cruise has had an “event” set up for it on Facebook. Often that is a way to connect with the very specific audience of people who are on the same cruise with you. Have a wonderful trip.
The advantage of the cruise sponsored tours is you don't have to worry about not getting back before the ship leaves port.
Besides Carol now retired's advice you could also look at the Cruise Critic site. There are Roll Call forums that are set up according to cruise line, ship, and specific sailing. Join your particular Roll Call to share your excursion with those who will actually be sailing with you. And learn about other excursions that are being shared.
I have done 21 ocean cruises and stopped taking many ship sponsored cruises some years ago.
The only reason that I might take a ship sponsored cruise is if the tour is a very long one with many miles of travel involved.
Private tours are always better with fewer people (usually) and cheaper (almost always). Just make sure that the company you book with is reputable and guarantees you not missing the boat.