We are traveling in Late September - will i need to book ferry prior to trip?
is there a slow ferry ? If so is it more scenic than the Hydrofoil?
About three years ago we took that ferry from Naples to Sorrento in late September. It's much better than going by Circumvesuviana train which is hot and full of pickpockets. We did not book anything. Just came to the harbor, look at schedule (there are many boats going there), bought a ticket and went. The scenery is the same from slow boat as from Hydrofoil (obviously). I am not sure if there is slow ferry.
Three years ago, we took the hydrofoil from Sorrento to Naples and back again. We got there right after breakfast and just booked the next one out. I recommend it, nice scenery, much better than the train. If you're prone to motion sickness, take something though. It can get pretty choppy in places.
Here's the current boat schedule, and no, there are no slow ferries on this route:
http://www.positano.com/en/ferry-schedule?from=napoli&to=sorrento
I doubt that you need to book ahead in late September.
Thank you to all who gave me advice. This was very helpful!