If you definitely desire having a sleeping cabin on the ferry, you should reserve in advance. Otherwise, you risk having to accept a reclining seat or, worse still, no guaranteed seat whatsoever. Much depends on the passenger load on the specific evening of departure. While May is not high season, the ferry should still have several hundred passengers (more on weekends). There is a limited number of twin cabins; once these sell out, you're stuck with taking a quad and paying a supplement for the unoccupied bunks. Securing space for the rental car is also a consideration, although I would not be too worried during May. I would reserve the sleeping cabin no later than 30 days beforehand.
Bear in mind that the Tirrenia "customary" ferry on this route ("Rubattino") accommodates 1,400 passengers and 700 autos; the SNAV ferry ("Sicilia" and "Campania") 950 passengers and 600 autos. "Rubattino" has approx. 288 cabins and 195 reclining seats; "Sicilia" and "Campania" a similar number (maybe fewer). However, if you sail on a night when Tirrenia's older ferry "Nomentana" ("Aurelia" class) -- is operating on a temporary basis, the accommodations are greater: 2,280 passengers, 645 autos, 320 cabins and approx. 600 reclining seats. You can determine which ferry operates on which days from the Tirrenia website. The "Rubattino" is classified "Nuovi Traghetti" whereas the "Nomentana" is classified "Nuovi Traghetti 'Aurelia'". There's not much difference between the ships in terms of interior accommodations, but they look very different from the exterior.
Here is more information about the ferry service options (pasted from another of my postings):
NAPLES TO PALERMO: both Tirrenia Lines and SNAV Lines operate daily overnight car ferries 365 days per year. Tirrenia has additional weekend DAYLIGHT sailings from the end of July through the 10th of September. A twin outside cabin ranges from 61.11 to 78.04 Euros per passenger, depending on season. Inside twins and quads available for less. A first class reclining seat ranges from 37.40 to 45.97 Euros. Deck passage (no guaranteed seat) ranges from 35.78 to 41.70 Euros. Port taxes are an additional 3.09 Euros per person. Ferry boat accommodates automobiles and trucks as well. Tirrenia ferry has air conditioning, restaurant, cafeteria, two cafe/bars, cinema, game room, card room, shopping, and large public salon. Tirrenia ships, built in 1999 and 2001 ("Florio" and "Rubattino") displace 30,800 tons, are 595 feet in length, travel at 23 knots, and can accommodate 1,400 passengers and 700 automobiles. There are 288 cabins and nearly 200 first class seats. SNAV ships, built in 1974 ("Campania" and "Sicilia") displace 27,000 tons, are 568 feet in length, travel at 20 knots, and can accommodate 950 passengers and 600 automobiles. Tirrenia crossing takes approx. 9.5 hours. SNAV ferries take one hour longer and cost less. Cabins and reclining seats should be reserved in advance, particularly during the summer. By contrast, Naples to Palermo by train takes as little as 8.5 hours by daylight Intercity train.
MODIFICATIONS TO NAPLES/PALERMO CROSSING ON TIRRENIA LINES
Pending the return to service of the Vincenzo Florio on the Naples/Palermo route, Tirrenia Lines is using one of its modernized "Aurelia" class car ferries (currently the "Nomentana", modernized in 2004) on alternate days with the Florio's sister ship, Raffaele Rubattino. Service remains daily overnight, in each direction.
The modernized "Aurelia" class are slower (19.5 versus 23 knots) and smaller ships (14,800 tons, 485 feet in length, versus 30,800 tons, 595 feet), but can accommodate more passengers (2,280 versus 1,400) and have an interior styling comparable to her larger sisters. There are a similar number of cabins, but no first class reclining seats. Instead, there is a larger number of second class reclining seats. As on the larger ships, there is a boutique, restaurant, cafeteria, two bars, cinema, reading room, videogames room, and large public salon.
The "Aurelia" ships date back to the late 1970's, and look very different from the more modern Florio and Rubattino.
The "Aurelia" ships will employ about 30 minutes longer to make the crossing.
The "Vincenzo Florio" will hopefully return to service on this route this summer.
Hotels near the Port: Pensione Bella Capri, right across the street (very basic, in a rundown building but modernized inside and with air conditioning). Much more "upscale" and more expensive is the business-oriented Hotel Mercure Angioino, off of Piazza Municipio and 300 yards from the Port. The luxury hotels ("Exclesior", "Vesuvio" and "Santa Lucia") along the fashionable Via Partenope are about 1200 yards distant.
www.tirrenia.itwww.snav.it