we visited pompeii last year, beautiful place. we took the cruise ship's tour but i think i would do my own thing next time. train station is very close to entrance and there are loads of english speaking tour guides vying for business. i think this is better because with the organised tour you had to go at everyone elses speed, i am a bit slow, and couldn't linger where you wanted.
bit of a tip though, bring lots of water cos absolutely no shade and the sun is lethal, wear very comfy shoes cos everywhere is cobblestones, and have a look around the cameo factory i got a beautiful cameo necklace as a souvenir.
I plan on taking a Mediterranean cruise in July, also to Naples. I was wondering, since it is so hot (as you described) would it be okay to wear shorts and a T-shirt? I don't know where, but I thought I heard that Italians (and other Europeans) resent the way tourists dress. Since I live in a very tourist oriented city, I could sort of understand. Also, I heard that the street criminals (pick-pocketers and worse) could tell who the tourists are by the clothes they wear and it might be a good idea to dress a little different, to avoid attracting their attention.
We went to the Mediterranean twice last summer. It was some Churches that you could not enter with shorts but the ship will tell you the dress code. Everyone was in shorts just about but the days we went into Rome etc and certain Churches were going to be visited. When you are in certain places there are hundreds of tourists to tell you the truth. My husband tried to wear a polo shirt sometimes instead of a T-Shirt but he likes his shorts! It is hot but your from a hot place too. I felt Pompeii was a must see and if time the Herculaneumn