I thought I'd describe our experience on the Tirrenia lines ferry from Palermo to Naples.
We had never done an overnight ferry so we chose it partly to replace a night in a hotel and partly just for a unique experience. After doing so, I would absolutely use this type of mobile lodging again.
Ticketing: We booked online directly with Tirrenia. There are numerous resellers so make sure you know who is issuing your ticket. Each adult ticket was €60 each and we added an outside cabin for another €94, so €208.20 all in with fees and taxes. So not exactly cheap but I think there's a lot of value involved in having a moving hotel.
Boarding: The worst aspect of the trip but I think a lot had to do with the season, construction, and just the fact we were pedestrians. We picked up our stored luggage and grabbed a beer at a place basically across the road from the ferry port. We could literally see the ferries lined up and google indicated a short walk to the gate. So we eventually tromp to the gate only to find it closed. So we walk, and we walk, and we walk until we find the open gate where cars and truckers are entering. There are no signs directing passengers so we just had to walk, asking dock workers which ferry is going to Naples. My Italian isn't great but it was useful here. Eventually a guy directing semi trucks stops us because he clearly figures two middle aged people carrying backpacks are gonna get killed in the dark, bustling port. He says "It's just up a bit further on the left". I'm relieved but after 50 yards we come to a corner and there's....nothing but a dark, empty pier. Huh?!? So we turn around and start to walk back. Along the chain link fence I notice a totally dark entrance... no signs, no lights, nothing. I tell my wife I'm gonna check it out and wind my way through the fencing to a construction trailer where I find two uniformed people. I explain we're looking for the Naples Ferry and he says "Great, that's us" pointing to the trailer. I'm massively relieved but I can't help but think how we only found them by luck. They were super friendly, checked our docs, chatted about our travels and generally exuded the friendly Sicilian charm/pride I had grown to love over the past weeks. We made our way to the ship, climbed up to the "hotel" desk and got our cabin assignment. Please Note that there is a new passenger ferry building under construction. I imagine that this process with be a LOT smoother in future
The Cabin: It was perfect. You can see pics on their site and it was just as advertised. A cosy little cabin with a tiny 3/4 bath, two snug beds with really comfy mattresses, plenty of outlets and a big window. It was clean, well-maintained and kinda cozy. I slept like a log
The Ship: It was nice, like a bare-bones budget cruise ship. There was a cafeteria line with good looking food. I kinda kicked myself for packing our dinner. They also had a kids game room and a lounge in the corner of the front main salon if you just wanted snacks and drinks. This salon is the main passenger area of the ship; lots of comfortable seating, TVs playing sports. Outside areas were just for standing, smoking and hanging out watching the activity in the port/the city as we got underway. The ship was basically empty since it was offseason; 50 passengers maybe. Ride was very smooth.
Arrival into Naples: I woke with the sun, so I was able to get out on deck and watch us enter the Bay of Naples. As we got closer, various announcements signaled the crew's desire to get folks debarked and cabins flipped. We packed quickly and hustled to deck to enjoy the view of the port and the city in the morning sun. We debarked, grabbed a cab to the Archeological Museum and...waited for it to open.
All in all, it was a fantastic experience and a great way to travel. You go to sleep leaving Palermo and you wake refreshed in Naples.