Is there a hydrafoil that goes from Naples to Positano?

yes but not always running. I ran into that last month. Rode down the coast on bus to Amalfi planned on taking HF back to Naples. That day there was sign of door of ticket office no trips today, no reason given. So if you plan to do that have a back up in mind. The bus can take you hours if the traffic is really heavy.

PS to fast with fingers. Here is site with timetables, boats etc,
http://www.metrodelmare.com