Was this in the days when the Dieppe ferry terminal was in the town, with Dieppe Maritime station? My research says that closed in 1994/the new out of town terminal opened.I caught the ferry between Newhaven and Dieppe on a number of occasions after that route took over the night ferry crossings from Dover to Dunkerque. The ferry was moored on the quayside the other side of the station building so you didn’t have far to go to get on board.
I used to try to make my connection off the Manchester to Newhaven Marine loco hauled service rather than the boat train from London Victoria.
As far as I understand (and has been said up thread) the ferry at Newhaven still uses the slip alongside the site of Newhaven Marine. But the combination of modern security/lack of demand/cost cutting means the only public entrance to the ferry terminal complex is at Newhaven Town.