Palmera by overnight ferry and back?

Do you mean Palermo in Sicily?

Oops. Yes.

Palermo has lots of interesting churches, chapels, museums, markets, etc., but I consider the Cathedral at Monreale (just outside of town, with city-bus service) the #1 sight. Right in town is the Capella Palatina, which also has magnificent mosaics. No doubt there's other Byzantine art to be seen.

The question is whether a single day in Palermo is worth the cost of two overnight ferries. Sicily is a great, great destination. The mosaics in Palermo are just a small part of Sicily's riches. Will you have time on a future trip to spend 2 weeks or so in Sicily? If not, and if the Byzantine art is super-important to you, it doesn't seem a crazy idea to me if you can sleep well on the ferry.

I haven't heard anything about that particular ferry. I've taken two other overnight ferries and found them far more comfortable than night trains.

Have you seen the seven mosaic sites in Ravenna?

Would it be possible to reconfigure your itinerary so you could begin or end your trip in Palermo rather than having to make a round-trip from the mainland to Sicily?

For me, personally, I would not do a one day trip to Palermo from Naples for a limited itinerary when there is so much to see and do in Palermo and surrounding area. If you cannot add days to your schedule I would then concentrate my efforts and interests on the mainland. For me, the time, money and effort to get to Palermo by ferry from Naples for one single day would not be an effective use of my time and energy but it may very well fit in with your travel frame of mind. I suppose it is doable, depending on ferry schedules and if you hire a private driver to get you to the specific sites you want to see but I also think it would be very rushed. If your schedule permits, please consider adding a few days to actually see more of Palermo area and enjoy more of what it has to offer. Byzantine art can be seen in Monreale, Cefalu, the Palantine chapel, and other churches. Very difficult to fit it all in one day. I assume you might be interested in concentrating that one day visit on the cathedral at Monreale which is spectacular. Make sure to include a visit to the cloisters at Monreale cathedral.

No, you need two nights at a minimum, three would be better.

I’d say at least 2 nights. But if you fly instead of taking 2 overnight ferries you basically have 2 nights! A flight in the morning on one day, that afternoon in Palermo, sleep, a full day there, sleep then fly back the next day.

As of three years ago (my last trip) there were direct flights Naples to Palermo for cheap. This might have changed due to Covid and you probably can’t tell yet what flights will be available once it’s safe. But it’s still a much faster option than the ferry even if there are few flights.