Well, I'll try to answer your question. The port is in the city. If you get off the ship and walk out of the port entrance, you are there. We walked to the train station and went to Pompeii and Sorento on our own, it is very easy to do. I don't know about tour buses in Naples itself... We didn't stay in the city.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013