Here is an article I did on this topic not long ago:
Med cruise : Mediterranean Cruising : a Primer
Western Mediterranean Highlights
These cruises often leave from Barcelona, on the eastern coast of Spain along the Mediterranean. The most popular ports on western Mediterranean itineraries include Monte Carlo (Monaco), Nice, Cannes and St. Tropez in France; and Naples, Portofino, Capri, Livorno and Civitavecchia (the port for Rome) in Italy. These cruises sometimes include the resort islands of the Mediterranean, which are better known to Europeans than Americans: Majorca, Ibiza, Malta, Sicily, Corsica. They sometimes include stops in Gibraltar or cross over to Morocco with a stop in Tangiers. The eastern boundary of a western Mediterranean cruise is the heel of the boot of Italy.
The main attraction here is the Spanish culture. Notable sites include buildings by famous Catalonian architect Antoni Gaudi. This is the man responsible for adding the word "gaudy" to the English lexicon. His striking buildings are similar to the artwork of Salvador Dali (another famous Catalonian); they often appear to be melting representations of otherwise normal figures. His most famous is the cathedral Sagrada Família that was started in 1882 and is yet to be completed.
The attraction here is the opulence, from yachts moored in the harbor to streets lined with designer shops. A stroll up the rugged coastline takes you to the palace of the Rainier family and the church where Grace Kelly was married and is now buried.
Any stop in France along the Mediterranean means you are on the Riviera and close to wine country. Soak in the local flavor, which is much more laid back than the hubbub of Paris. St. Tropez is a decadent yachting town with more nightlife than daytime activity. Marseille and Nice are port cities where you are advised to watch your pocketbook.
The stops along the west coast of Italy are the highlight of a western Med cruise. In the south is Bari, one of the oldest and dirtiest cities in Italy. Moving up the coast is Naples, where the recommended tour is to ancient Pompeii, the Roman city that was almost completely preserved in ash from the eruption of Mount Vesuvius 2000 years ago. The delightful port cities of Portofino and Portovenere bring back the charm of 18th century Italy where sidewalk cafes are cooled by sea breezes, and fishing is still a way of life. Cinque Terre is a walk along a natural national preserve on the Ligurian coast.
Livorno is the port city for Florence and Pisa. You can catch a train to both cities if you leave early, but the best bet is a ship's tour that will get you to all of the sites and back to the ship in time for sailaway.
Civitavecchia is the port city closest to Rome, and is often the end-point for this itinerary. Getting to the city requires a transfer from the cruise, the ride being about 90 minutes. If your cruise begins or ends here, plan to spend at least two full days seeing the city.
Other than Malta, which is architecturally fascinating, the resort island stops are generally pretty boring for Americans. As the only sunny places close to Europe anytime other than summer, they are tourist traps with discos and swimsuit shops everywhere. The beaches are rocky sand and the water is cold. (By the way, Mallorcan pearls are not real pearls; they are macerated pearl material held together by glue and polished to a shine.)
Eastern Mediterranean Highlights
Italy on the west and Turkey on the east frame the Eastern Mediterranean. Some itineraries will expand to Israel or Egypt, or north into the Black Sea. The highlights of an Eastern Mediterranean cruise include the Greek Isles, Venice, Dubrovnik (Croatia), Istanbul and the ancient city of Ephesus in Turkey.
The beginning and ending ports for most Eastern Mediterranean itineraries are Civitavecchia (Rome), Athens (Greece) or Istanbul (Turkey). Remember, if you are flying into Rome, allow at least two full days to see the city.
Athens can be seen in one full day and night, the highlights being the Acropolis and the surrounding area known as the "Plaka" -- a square mile of pedestrian streets with souvenir shops punctuated by restaurants.
Nearby you will find the ancient "Agora" ruins -- worth a few hours of exploration. If you have time you can visit the National Museum of Athens to see marble statues and artifacts like jewelry and armor from the Hellenic period. However, you we see plenty of such artifacts in other destinations on the cruise.
The legendary Greek Isles would require an entire article. "Must see" islands include Delos, the ancient capital of the Hellenic Empire before Athens and once a trading center for all seagoing people of the first millenium BC. The island's many ruins include examples of Greek, Roman, Phoenician, Egyptian and Minoan architecture.
Santorini is the famous island with the city of Thera high atop a cliff, above the caldera of a volcano that erupted around 2500 BC. This eruption is said to have been one of the largest known to history. Your ship will anchor in the caldera and tenders will transport you to the bottom of the cliff below the city. From there you take a cable tram or a donkey ride to the city above.
Although there are thousands of Greek Isles, among the most famous are Mykonos, Rhodes, Crete, Cyprus, Naxos and Lesbos. Crete is known for the Minoan ruins, some of the oldest in the world (2000 BC), and for the tales of the Minotaur and the labyrinth. Little is known of this civilization because they did not leave any writings behind. It is believed the eruption of Santorini created a tidal wave that wiped it out.
The ruins of Ephesus, near the port city of Kusadasi, Turkey, is one of the seven wonders of the ancient world. The city -- referred to in the New Testament as the place where Paul preached to the Ephesians -- still has the ruins of a Roman theater and the façade of a famous ancient library. Nearby is the house where James the lesser allegedly brought Mary, mother of Jesus, to live out her years. His crypt is near Ephesus in the Turkish city of Selcuk.
Venice, Italy, is one of the most beautiful and intriguing cities in the entire world. No cars or other vehicles are allowed there -- only pedestrian traffic and boats. The city is built upon hundreds of tiny islands connected by bridges. The main transportation system, the "Vaporetto," is a network of bus-like ferries that run throughout the city on regular schedules. Many cruises begin or end in Venice. Ideally, you should set aside at least one full day to see Venice. The best cruises set aside two days to visit.
Dubrovnik, Croatia is on the Dalmatian coast in what was once Yugoslavia. This walled city that juts out into the Adriatic Sea was a formidable trading and military rival to Venice in the days of Marco Polo and city-states. Among the city sights is the world's first pharmacy, still in use.
Becoming a more popular stop on cruises, this most northerly of the Greek Isles is unlike its southern sisters. While most of the arid and treeless Greek isles are in the Aegean Sea closer to Turkey, Corfu is northwest of the Greek mainland, off the coast of Albania -- and it is covered with vegetation.
This famous ancient city is known as the place where Europe meets Asia. The dividing line is the thin strip of water known as the Bosphorus Strait that leads to the Black Sea. Some people think the city is fascinating, while others find it sprawling and dirty. The most important sites include the Istanbul Archeology Museum and the Hagia Sophia Mosque, once the largest in the world.
Other possible eastern Mediterranean ports of call include Alexandria, Egypt, where you must see the pyramids of Giza and the Sphinx. Cyprus is an island not far from Israel, half Greek and half Turkish. The two countries do not like each other and you must travel back to the mainland in order to get from one side of the island to another. Israel stops include Haifa, and if you are lucky you have time to visit Jerusalem and the Dead Sea.
Of all cruises in Europe, the ones that stand out in the memories of most people are the Mediterranean cruises, especially the Eastern voyages with visits to Venice and the antiquities of Rome, Ephesus, Athens and Delos.
The warmth of the Spanish Islands and southern Turkey, which draw plenty of Europeans, is a far cry from what Americans want to see. So selecting a cruise line that normally caters to U.S. tourists -- such as Carnival, Celebrity, Holland America, Royal Caribbean, NCL or Princess -- is the right choice. You get all the advantages of paying in dollars and your tours and shipboard activities are conducted in English. You get plenty of chances to try the local cuisine, but you are not forced to pay exorbitant prices every time you get hungry.