Aida, Costa, Carnival, MSC, RCCL, Princess - nobody went spa'ing in the buff.
To me it seems everyone does what he or she sees fit when entering the non co-ed sauna but not the co-ed spa as you say ... Didn't see that happening cruising with a Spanish and Italian crowd (nude beaches galore). Even on the german-based Aida ships (Germans are known for their nude sauna habits) there was a seperate FKK bereich (nudist area) on top deck far away from others and nobody walked around in their birthday costume in the spa 🙂
I do however shower nude at the spa's non co-ed showers (refuse to shower with any clothing - that's just stupid) and it does seem to encourage others to do so, too.
Now if we could all start going full frontal in the spa maybe we can create a revolution in cruising ... lol