Positano to ischia

There are ferries from Sorrento, Naples, and Pozzuoli.
The Sorrento to Ischia is operated by Alilauro.
More than one company operates the crossing from Naples.
Travel time varies depending on speed of ferry (some are high speed hydrofoils) you choose and port of departure. Crossing times range from 50 min to 1h30min.

If you travel in the very near future be aware that there has been an earthquake in Ischia yesterday, therefore travel to the island may be adversely affected by the rescue efforts. It's probably not a good idea to go there for a few weeks at least.

There is an 8:50 ferry from Positano-Ischia in high season.

https://community.ricksteves.com/travel-forum/italy/ischia-earthquake
Small EQ near Naples but one fatality and damaged buildings in Ischia.

I heard about earthquake, so sad! I'll be going to positano sep 22 thru 27th, hopefully all will be good then.? Is that high season? Thank for info!