So the boat docks and takes off at 7 AM and 7 PM? You won't get off until near 8 AM, and would probably want to be back near the port around 6PM? That gives you ten hours. That could be difficult to do much outside of the city itself.

Pompeii is one of the world's most important archeological sites. Because the type of ash that covered it was soft and not destructive it just covered the city. It was not hard lava. Thus, the city was not destroyed, the graffiti on the walls remains as it was, you walk into a kitchen from 79 AD, and the pots and pans are still hanging on the walls. It's as if time just stopped. It's a remarkable experience. Maybe the "reviewers" didn't have a nice trip because they had poor transportation, went on a very hot day, weren't prepared for the sun, went when it was super crowded, didn't have a guide and didn't know what they were seeing, they were stuck in the super long lines?

That being said, I'd be a little anxious because you only have about 10 hours. Pompeii is not that close, and you can get stuck in rush hour traffic both going and coming back. Forget the Amalfi Coast, or Capri.

That's up to you if you can take that kind of rush without being too worried. If you do, consider having a driver waiting at the dock to take you there and back, and taking a 2 hour highlights only cut-the-line tour, and get back to Naples ASAP.

Naples is a marvelous city. The archeological museum contains many of the artifacts taken from Pompeii. The Veiled Christ in Capella Sansevero is one of the great masterpieces of sculpture. No one could fail to be moved by the artistry that went into it.

Then of course, pizza, but make sure you know the better places (Sorbillo, 50 Kalo, DiMatteo, La Notizia, etc. Just don't go to Al Presidente. Da Michelle, well, let's just say if it wasn't for the movie....). Sorbillo would be great because it's in really old Naples, a part of the city you should really see. You'll need a cab to and from because otherwise you'll get lost.

You can have it leave you off at the plaza in front of the opera house, Piazza del Plebiscito, then walk down to the Lungomare and stroll along the waterside, looking at Vesuvius in the background. It's one of the most beautiful walks in Italy, and you can take it in just before grabbing a cab for the short ride back to the boat.

Given the lack of time, I'd err on the side of not feeling too rushed, and not having to worry about missing the boat, so you're going to need to think this through.