I haven't been on ANY balcony on ANY ship that I would consider private enough to go "au natural"! For instance, I remember a man next door to us that liked to soak in the tub on his balcony in the nude. When I sat in my chair with my back to the glass door of the cabin (out of the sun/wind) I had an unobstructed view of him... kinda shocking the first time! I'm sure he never suspected that anyone could possibly see him when out to sea! FYI, those balconies just aren't that private. People can look around/between the partitions and over from the top.
Of course, if that doesn't bother you... go for it.