There are regular ferries from Naples to Ischia and one or two a day from Sorrento in the summer months. There are ferries between Capri and Ischia but they are not frequent, and even if they were, you are not going to see much of either place. There are also regular ferries from Sorrento, Naples and some Amalfi Coast villages to Capri. We've had three long holidays in Ischia and three day trips from Sorrento to Capri and barely scratched the surface.
Ferries to Capri