I believe that this is part of the Stazione Marittima cruise ship terminal. The classic circular "porthole" style windows hark back to the golden age of maritime architecture.

Again Napoli is one of the largest passenger ports in both Italia and Europe. According to Wikipedia there are ten berths in the cruise terminal with an annual capacity of 1.5 million. According to the website of the port itself, there were 1,141,686 cruise passengers in 2018, up from 993,073 in 2017.

However this pales in comparison to local traffic on the Bay of Naples. (Bearing in mind that Napoli is the usual starting point for trips to Capri (including us) or the Amalfi coast, amongst other places.) For both of the years above the number was north of 6.5 million. You can add another million or so for roll-on / roll-off vehicle trips.

All in all, this is one busy not-so-little port. For tourists on cruises, this is one of their first impressions of it.