According to the WHO, tap water is safe for consumption for 100% of the country.
Tap water is drinkable and safe in most of Greece but not always in small villages and on some of the islands. Always ask locally if the water is safe, and if in doubt drink boiled or bought water. Even when water is safe, the substances and bacteria in it may be different from those you are used to, and occasionally can cause vomiting or diarrhoea. Bottled water is widely available, but think about the environmental considerations when you opt for bottled over potable tap water.
According to the CDC tap water is generally safe for consumption.
According to the Numbeo water drinkability and accessibility index, tap water has a score of 75.14 / 100 (100 being best).
Numbeo considers this score to be "High"
Greece is a member of the European Union which ensures tap water safety across its countries.
1 adult could save about 613$ per year in Greece by drinking tap water instead of buying bottled water.
๐ณ Drinking tap water would also help with reducing your plastic footprint and improve environmental pollution.
Like in most countries, water quality and accessibility differs from location to location, therefore it's the best to check the information available on that place.