Passengers on both the Naples/Palermo and the Palermo/Naples Tirrenia overnight ferries scheduled to depart at 20:00 on New Year's Day spent the night in their cabins on the ferries moored in the ports of Naples and Palermo, due to Force 8 sea conditions which forced the cancellation of both crossings. Both ships departed in the early morning of January 2, roughly 10 hours late.
The news reports did not mention the fate of the competing SNAV ferries, but I presume that both also "spent the night" in port.
Maritime crossings to the "minor islands" (e.g. Aeolians, Ustica), were also cancelled, stranding tourists and locals for a couple of days. This often occures in winter.
There are two facts to remember about this incident:
Ferry crossings are subject to being cancelled when sea conditions become dangerous. This occurs primarily during the winter, and more often for afternoon/evening departures.
The Palermo/Naples ferry had 100 passengers on board. The ferry accommodates 1400 passengers. It is often stated that advance reservations for this ferry crossing are unnecessary during the winter, as this occurrence evidences.
Be guided accordingly. Have a "backup plan". Be patient.
Here is the news story, from La Repubblica:
Sicilia. E' partita stamattina alle 6 da Palermo per Napoli la nave Rubattino della Tirrenia che ieri avrebbe dovuto salpare alle 20, ma che è rimasta in porto a causa delle cattive condizioni del mare. Sempre nella prima mattinata ha lasciato il porto di Napoli il traghetto Janas per il capoluogo siciliano. I circa cento passeggeri imbarcati ieri a Palermo hanno trascorso la notte sulla nave. Attualmente le condizioni nel Tirreno meridionale sono quasi proibitive, con il mare Forza 8. Da Palermo sono stati sospesi i collegamenti con Ustica e sono rimaste in porto anche le navi cisterna che avrebbero dovuto portare acqua potabile nelle isole minori.