Lying equidistant from Naples (about 25 km [16 miles]), Ischia and Capri stand like guards at the main entrance to the Bay of Naples, with Ischia to the west and Capri to the south, while Procida is like a small stepping-stone halfway between Ischia and Capo Miseno (Cape Misenum), on the mainland. The islands can be reached easily from various points in and near Naples, and the port of Pozzuoli is the closest to Ischia and Procida. Nevertheless, most people head first to Capri using hydrofoils from the main port terminal in Naples—the Molo Beverello. Ferries are far less frequent, and leave from the nearby Calata Porta di Massa. There are also boat connections between the islands.
You should buy a single ticket rather than a round-trip, which would tie you to the same line on your return journey. Day-trippers need to remember that the high-season crowds on the last ferries leaving the islands make this crossing riotously reminiscent of packed subways and buses back home; in addition, rough bay waters can also delay (and even cancel) these boat rides.