Find a map of the area and you can see why taking Waterways would be more advantageous than using Land access. BTW, years ago we did a Grand driving trip of Europe - Sweden was the most northern part we went and Spain the most southern part. The trip took slightly over a month, did not cover Eastern Europe, nor Portugal and Greece, but included all countries in Europe.
St. Peter*****urgh - the major attraction is Hermitage. Virtually all cruises would stay overnight at St.Perter*****urgh to give passengers enough time to visit Hermitage and the city. Not sure if this is correct, Hermitage is the largest museum in the world, supposedly.
You would enjoy Italy A LOT MORE when you can do a land trip.