Misleading.

Use google to check what currently is plowing via the sea as boat/ferry. DO be aware that many of these ferries are since a yr or 2 only for people + cars-and some only for trucks. You will only discover that when you start to try make a booking as pax-only.

Catania is on the so-ea side of Sicily-not a very obvious destination to send ferries to from the Napoli area.

Cagliari is on Sardinia and still has quite a few ferrylinks with several ports in mainland IT.

There was a Salerno - Catania route run by Grimaldi Lines but it is not currently in operation. Other routes such as Salerno  Palermo on Sicily operate taking foot passengers as well as vehicles. I see no discounts available.

Why worry about ferries when you can travel by train from Naples to Sicily including a crossing on the last boat train in Europe - Yes your train goes onto the ferry.  Reservations are as little as 3 euro via OEBB.

Also night trains are available.

Alternative to the relatively slow InterCity service 2 or 3 times per day there are slightly faster Freccia trains to Villa San Giovanni and then a short walk to the ferry across the Messina straits to connect with local trains.

Thanks for the replies. I will use the trains!

You need to be a little careful. Using regional Trenitalia rather than IC the Rail Planner will clearly show the ferry segment as part of the trip with correct timings. It does NOT flag up that a booking (€2.5) is required. Without this you will not be allowed on the ferry and may miss a connection.