Barcelona had a post bus which took you the mile from the dock to the main square. The hop-on hop-off bus has 3 routes that covers all the highlights.
In Rome, as Geordie said, take the port bus and the train into Rome. In Rome, the subway is easy to use to move around the city.
Naples is the one place we took a ship's tour mainly because there was one that was exactly what we wanted to do.
That area of France is generally easy to walk around - stroll along the seaside, lots of cafes, restaurants, etc.
Best advice is to pick one or two things you want to see rather than trying to see it all. It also leaves you an excuse to go back again.
Also check Fromers on-line - they often have walking tours posted, for example there is a great one for the ruins in Rome.