I this summer I made a cruise with my family (my wife, my sons and one aunt) on NCL Gem and one of the stops was Naples. To visit Pompey, take the bus at the bus stop in from of the cruise terminal to the railway station (10 minutes away) and then take the train to Pompey Scavi. The train station is next to the entrance to Pompey Scavi so the bus is not needed. Inside the Cruise terminal the is an Italian Tourism desk that will give all the help and maps needed to you visit.
Enjoy the visit to Pompey and Naple
PS - careful with taxis and ask always the fare to the driver before taking the taxi (usual precaution in big cities