Celebrity Cruises used to permit topless but only in designated areas. It was an area above the pool deck. That said, there was always a parade of husbands, in pairs usually, that needed to see boobs and would make the rounds.
I remember Cruising on the Carnival Holiday in August 2003. They did have a topless deck (Sun deck) with quite a few participants, but there was no total nudity on that Ship. I don't recall seeing any similar activity on other Ships we have sailed on.