Wait, did you actually find overpriced water in Greece? I ask because they have a law regulating the price of bottled water: https://greeklandscapes.com/prices-in-greece/#:~:text=BOTTLE...).

In my experience, I was able to purchase 0.50€ water at airports in Athens, Mykonos and Santorini. One trick that I saw some merchants use was to not to sell bottled water at all, or claim it had ran out, and that they only had 7€ Gatorade or whatever.