Try Googling "captain barefoot skiathos" and you will get a lot of information about nude beach options on Skiathos. There is a lot of choice depending on whether you want exclusively (Well mainly nude) or clothing optional where you may be in a minority. When I visit the Greek Islands I print out the relevant sections of Captain Barefoot's guide to take with me. We have found that many quieter beaches are effectively C/O providing you are sensible.
We are going back to Skiathos in Mid-May 2016 for 2 weeks - we are hiring a car this time and hope to visit as many beaches as possible to check if they will tolerate nudity. I understand that many quiet ones we are thinking of (though not "officially" naturist) accept the naked body.
Anyone else going there at that time?