I just spoke with a joung lady, bourn in Sorrento. She said, it is very simple to go to the "cruise terminal": jump out of the train Circumvesuviana and take the minibus to the port Marina piccola . The cruise ships are too large to reach the port. They roadstead. Hope you know this word.I learned it in my dictionary. In german we say "the ship is auf Reede". That means: the ship anchors outside the port and the embarkation will be made by the "rescuevessels" .
If you want to go to the port by walking, mind, that there is a huge staircase between the rail station and the harbour. It takes about fifteen minutes and it is a hard work with your luggage.
My female friend said: take the bus.