How large is your ship? I have never cruised, but I've read many comments here about not assuming you'll get off the ship quickly if you're not on a cruise-line-operated excursion. Therefore, I wonder how realistic it is to expect to start an excursion at 7:30 AM.
I also wonder about the possibility of limiting the excursion to Ravello or Ravello/Amalfi and using the train for transportation from Naples to Salerno (generally at least 2/hour take no more than 47 minutes), followed by a taxi/private driver from Salerno to and from Ravello (and Amalfi if desired). That would be just a 35-40 mile round-trip, though the drive would no doubt be slow along the beautiful coastal road.
I have no idea whether this makes sense--just putting it out there for the knowledgeable folks to comment on. Any extra time could be spent getting pizza in Naples and/or seeing part of the Archaeological Museum there.