The Captain is grateful for the first report concerning naturism on Poros. (However, The Captain would also welcome up to date reports.)
There are no official nude beaches on Poros. However, it is possible to find places to be nude. Hire a bike or scooter, drive north and pass the beach Russian Bay - which has a drinks kiosk and is a clothed/topless beach. Approx 800 m after Russian Bay you will find the first beach. You can see it from the road. The beach is about 100m long, not sand but "rolling stones". The beach is okay.
The next possibility is about 500 m after this. You will find a small farm with goats on your right hand. Down to the left you will see a bay, looks quite nice from that view. When you go down you will find a lot of garbage on the first beach in the bay, but follow the waterline to the left, and about 300m and you will find a suitable place. Not a beach exactly, more stones and cliffs but OK.
On the right is a longer beach which is rarely visited and is used by occasional nudists. A bit stony but the water is wonderful and it is quite private. There are consequently no facilities and this is best approached by scooter.
The Captain has had an email from a gentleman who says he cleans this beach twice a year. He says it is very quiet, especially in May/June and September, and most of the time he is alone there, but small sailing boats call there for nude swimming, which is fine but for the garbage they leave behind. Nonetheless he gives the beach a 70% rating.
Perhaps the gentleman has done a good job as the beach and water are clean. Nude swimming from yachts is common but I disagree that these boats are a source of litter; most yachties are conscientious about this. (June 2015)
Enjoy. There were several small places in the same area too. Bring your own food.
The beach is called Ntana and is reached from a footpath that leaves the road about a mile from Russian Bay. It is signposted, but quite small.