Greece and Turkey
These may be the only 2 that are good to use cruises, as the geographic set up works - i.e. the cities are right next to the ports where the ships dock.
Med cruises - there are several variations of such
is generally NOT a good way to do because the city center is very very far from the port - it usually requires an hour to 2 hours train or bus ride to get into city... enough said.
So, the key is WHICH part(s) of Europe you want to see, largely determine whether cruise is a good choice.
It cannot be generalized.