You can catch the Circumvesuviana train from Naples to Pompeii (the site is within yards of the station exit) then onto Sorrento (same train) I can't remenmber the exact price, but it was around 9euros return to Pompeii. The ship docks near the ferry terminal where you can catch the ferry or hydrofoil to Sorrento, Capri, Ischia......
Lots of cruisers DIY in Naple.
We wnet to pompeii we enjoyed every min i would say some thing not to miss but it is very hot going round the site plenty of suntan lotion and a hat is neede.
I would go on a trip instead of going to naples to much traffic and not a resort.