Koufonisia (or Koufonissia, even the islands own website doesn’t seem sure) is in fact three islands. Keros is uninhabited while Kato Koufonisia lacks mooring options and also almost uninhabited. So head for Pano Koufonisia.
Pano Koufonisia covers just 5 square kilometres with a population of around 400. The main moorings are at Parianos, a short walk from the main ferry quay and town.
There’s a good choice of beaches, most along the south coast. A good way to get there is to take the local water bus from the ferry quay which runs up to the east end of the island. You can either take the boat back again or walk along the cliff top, vising other breaches along the way. It’s only abuot 3.5 miles.
Beyond the beaches there’s not a lot by way of attractions. A small folklore museum has been formed from donation from the inhabitants. There are a couple of windmills. There are various caves and natural pools, though none very remarkable.
Of perhaps more interest to yachties is the boat yard. This is the only one for miles around. This probably explains the huge number of fishing boats. In terms of boats per head of population, Koufonisia apparently holds the record for Greece.
There are a few tourists, largely Italians for some reason, now encouraged by the ferry service from Piraeus. However, it’s not a busy place as there’s little to do but lounge on the beach. And what’s wrong with that.