I believe we had a report within the last 2 years from a traveler who
had purchased a ticket from Company X and was ushered onto a ferry run
by Company Y. I don't know how common that is, but I thought it was
interesting.
That may have been my report. Here's an excerpt from one of our days gong from Sorrento to Amalfi Town;
We found our dock, and the boat was there but it was deserted and so we sat nearby to wait. Our ferry was leaving at 10:20 and the competitor’s ferry-Caremar was leaving at 10:00. At about 9:55 there was still nobody at our ferry to begin boarding and a guy with limited English came over and gestured that the ferry was leaving. We showed our tickets and gestured that we were on a different boat, finally in English, he said the boat had changed and to get on. So, we hopped on the Caremar ferry hoping we were doing the right thing. As we were nearing Positano, OUR boat went speeding by, full of people and arrived in Amalfi well before we did. We had watched this guy intercepting many people and directing them to his boat and we have no idea why, because there was no financial incentive as we hopped on and off with no issues. On the way back, we befriended a bunch of Brits waiting on the dock and they had the same experience, plus their return tickets were for Caremar, but the time scheduled to leave was the same as ours. We all hopped on the Alilauro boat, and nobody had any issues.
And two days later from Capri back to Sorrento;
As with yesterday’s ferry experience, nobody seems concerned with what your ticket says. People with tickets for an earlier, later, or even a different ferry company were hopping on, and the crew didn’t seem to care as they took tickets without looking.
I'm not going to advise people to plan on this, but it was what happened to us.