My family of 4 has done 3 cruises on the Disney Magic in the Mediterranean. We had wonderful experiences on all of our private shore excursions, and wouldn't hesitate to recommend any of the companies we used. Note that in Italy, drivers are just that -- they can transport you from place to place and provide lots of information, but they can't go into sites with you. For help within attractions, you need a private guide (which we booked through the tour companies we used for the Vatican and Pompeii).
Civitaveccia/Rome
On the cruise that had a port stop in Rome, we did a private Rome Highlights tour with Rome in Limo that included stops at many sites (including but not limited to the Coliseum, Piazza Venezio, Panthenon, and Trevi Fountain) and a guided tour of the Vatican Museum, Sistine Chapel, and St. Peter's Basilica.
On the cruise that finished in Rome, we did the TIvoli Gardens tour with Joe Banana that picked us up at the port, stopped at Villa Adrian (ancient) and Villa d'Este (Renaissance), then dropped us off at our hotel near the Rome airport. (At our driver's suggestion, we stopped by the hotel first to drop off our luggage since it was nearly on the way to Tivoli.) Our driver recommended a restaurant to us that was our favorite of our entire 2-week trip, in a building adjacent to an ancient church. This tour is also offered as a shore excursion.
Naples/Salerno
We did the private Gastronomic Tour with Pompeii with Joe Banana. We visted a limoncello factory (with tastings of 3 kinds), an olive oil orchard (with tastings of at least 20 different kinds), a restaurant up in the hills (where we selected fresh toppings and made our own pizzas), and Pompeii (where we had a private guide arranged by Joe Banana).
Marseille
We did a private tour with Provence Explorer that visited Avignon (where we toured the Pope's Palace on our own with an audio guide) and Chateau du Pape (where we had a private tasting of 6 different wines).
Barcelona
On the two cruises that ended in Barcelona, we used Barcelona Day Tours. On one, we did a private half-day Girona tour that dropped us off at our hotel in Barcelona. We enjoyed Girona, but in retrospect, I wish we had done the full-day tour that also included the Dali museum in Figueres.
On the other cruise that ended in Barcelona, we did a small-group half-day tour that stopped in Monjuic and Parc Guell, along with the outside of Sagrada Familia, and dropped each group off at or near their hotel. This was fine for the price, but due to the need to wait for everyone to regroup at each stop, we didn't see as much as on a private tour.