vandelay2001, it's pretty hard to drag vital information out of you. How many of the guests use two canes, a wheelchair, or cannot have nuts or gluten present at their lunch table? Do any of them think they are going to get to wade on a beach? This target area is absolutely jammed with tourists from May to September. In case you miss the ship departure, what is the next port? Are you making plans like this for six ports?!?
BTW, with 20 people and limited time, you may well want the banal but satisfactory hot-cafeteria setting inside Pompeii. They are set up to serve mass-food quickly. They also have more than one bathroom. There is no food inside Herculaneum, and you cannot re-enter either excavation on a same-day ticket. There are limited restaurants near Herculaneum, but our European slow-food lunch there took nearly two hours, in a very informal, formica-table setting.
You need a guide, just because you must have someone with the authority to order souvenir buyers to get ON the darn bus, so that they can meet the timeline (unlike those ship tours you talked about) that you have set for the day. You also have to prevent independent-minded guests from going somewhere other than the place you have planned, for their lunch. Is the Naples port stop a Tender anchorage, or tied up on shore? What is the actual required re-boarding time, not the "sail-away" time? Will some of your guests be delayed in getting ashore because they have lower-budget cabins that exit the ship later?
I would also hope a guide knows a system for buying 20 tickets quickly, all at once, instead of 10 to 15 people fumbling for Euros that they don't have.
You need to collect a lot more information, maybe from a printed guidebook. Sorrento is a perfectly nice town, but it does not "look like" Positano, which you seem to be describing as a photo opp. Or maybe you mean Cinque Terrre? I don't mean this as unkindly as it sounds, but Sorrento is more like a (very large) cruise shopping mall, but with a cliffside view. It's not like a tiny Italian village with old widows in black, shopping with wicker baskets on their arms. But Positano and Amalfi are not like that either, they're just older (i.e. pre World-War II) and prettier.
You don't have time to go to Positano, partly because of traffic delays, but also the total distance and parking restrictions. I imagine there is a ship tour that spends 90 minutes in Pompeii and does go to Positano, but they do it with 5 busses, almost every day of the year, for one cruise ship or another!