I'll say, you can't go around Italy, it's a peninsula!!
Sorry, just kidding. I suggest going to our itineraries page here: <http://www.cruisemates.com/articles/cruiseguide/shiptour/destinations/med.cfm> and looking for ships in Italy (Renaissance is a good bet). Then go and read the ship reviews for the cruise lines.
I'm not aware of any cruise lines that offer cruises only to Italy. Your best bet might be Princess's "Grand Mediterranean" itinerary which operates between Istanbul and Barcelona. This itinerary calls at three Italian ports -- Venice (two days with an overnight stay), Naples (for Capri and Pompeii), and Livorno (for Florence and Pisa). There's also an optional two-day "overland" tour of Florence, Rome, and Naples that leaves the ship in Livorno and rejoins it in Naples. The remaining ports of call are Monte Carlo, Athens, and Kusadasi (for Ephesus), and overnight stays in Istanbul and Barcelona permit time for sightseeing in both of those cities.