While in many of the big cities, towns, and villages, the tap water is okay to drink, that isn’t the case everywhere. Always ask if the tap water is potable before going ahead. If you can’t ask anyone, be on the safe side and simply use bottled water.
Especially in the islands where water is often ferried in or pumped from wells, but also in certain areas in mainland Greece, the water is either too high on minerals or not filtered enough to be safe for consumption. However, the water is safe for cooking and washing as it is not contaminated by microorganisms or bacteria anywhere in Greece.
This article briefly talks about what makes the City of Florence such a magical place where people wander about can experience awestruck, and how is this a widely accepted truth.