Taking the road out of Vathi, you will see a track off to the right signposted Fikiadha. It's a small pebble beach situated in a deep bay - a bit untidy but the walk is the attraction - you can go completely nude for over four hours in the direction of Plati Ghialos. Amazing!

Once you reach the beach (90 minutes) you can have a quick swim and then continue through the olive groves and along the coast. A Barefoot reporter did the walk twice in mid-June (year?) and met no-one. He rated this beach 100%.
I was in Fikiada two weeks ago and its amazing, we did all the walk naked from Paris Gialos and its wonderful. The beach it was quite empty and we spent all day naked. There were other 3 couple naked not far from us.

July 2023: As a sailor, I was already very familiar with Fikiada Bay. The fact that it is only accessible by yacht or on foot makes this and other bays in the very south of the island of Sifnos interesting for naturists. However, due to the strong wind, neither Fikiada Bay nor Chlakopo Bay were well suited during our visit.