FL,
It would help to have a bit more information about your overall Itinerary. Where are you going after the Pompeii visit?
You might consider taking the Ferry from Ischia to Sorrento and spending at least one night there, as that's a convenient location for getting to Pompeii. AFAIK, there is at least one Ferry company operating on that route - https://ca.directferries.com/alilauro.htm . That will be far easier than taking the Ferry to Naples, taxi to the train station and then train to Sorrento.
Choose a hotel in the main part of Sorrento, and then just walk to the Circumvesuviana station and take the short ride to Pompeii. You might consider hiring one of the local guides for your Pompeii visit, as you'll learn more and it will be a much more interesting experience. It would also be a good idea to have a look at the RS Italy guidebook, as there's lots of information there on touring in that area.
An alternative idea would be to take the ferry to Naples and spend at least one night there. Take the Circumvesuviana from there to Pompeii. Be sure you're wearing a money belt as you may encounter pickpockets.