Because we bought our own tickets, from the ferry company on the pier in Sorrento, they almost demanded that we select and buy a return ticket at the same time. That was also late May, already very crowded in this area. I don't dispute the idea that the boat crews are careless about denying boarding to people without the right ticket. But to me, that increases the importance of having a return ticket, so you can wave it around ("talking Italian with your hands") if they select you for denied boarding.
Both our (two visits) return ferries were absolutely jammed, because they were among the last of the day. It was more like a subway car than a boat ride. And it was a bigger boat than we had arrived on. We had to look for the right pier on Capri, because the bigger boat had to use a bigger ferry slip than the one we came on. This resulted in extra walking time, maybe 1/4 mile.
I also noticed that the ferry window for company A in Sorrento would not tell us "Hey, you could leave 25 minutes sooner with company B, over there!"
Note that Capri is very big, and significantly pedestrianized. We happened to be on foot for our long approach to the return ferry. But especially if you are use a transportation mode (taxis can't reach every part of Capri, for example), that you allow enough time to get to your scheduled ferry.