Water in Greece

Yes and yes.

Water is safe. Like many places though, for drinking, you may not like the taste (My local water is just fine, but I drink water from a filtered system, to remove the chlorine at least) Many might order a bottle of water with dinner, or get bottled water when out for the day. On some of the islands, the water might also be a bit more processed, to remove salts, or might even be shipped in.

The mainland is safe but I was told not to drink the water on the islands.

I don’t know where you may have heard that water on the Greek islands is not potable. It certainly was drinkable during my visits to Rhodes, Kos and Corfu.

Santorini, Naxos, Mykonos and some other islands use desalinated water which is drinkable, although some prefer the taste of bottled water over desalinated tap water.

The water is 100% safe everywhere in Greece, on the islands as well as on the mainland. It is treated the same way as everywhere in Europe and the US.

In mountainous areas, and even in mountain villages on the islands, it's spring water that's purer and better than the water you drink every day.

But if you prefer mineral water because of the taste, buy bottled water. This will also be beneficial, given the water shortages in Greece due to climate change and the impact of tourism.

https://smartwatermagazine.com/news/smart-water-magazine/water-crisis-hits-greek-islands-amid-tourist-boom

It depends on which island you visit. For example, in Paros, tap water contains marble and therefore locals have to buy bottled water.

For example, in Paros, tap water contains marble, and therefore locals
have to buy bottled water.

Where does this information come from that the water in Paros contains marble? Can you provide your sources?

I've spent several weeks in Paros over the past 15 years, and I've never heard of this. I've also spent time in places right next to the old marble quarries where the tap water was perfectly drinkable. I have Greek friends living in Paros who have never mentioned this to me. Generally speaking, I've seen locals drinking tap water, including families with children.

I've checked the water analysis records taken in several places in Paros, and I don't see any excess of calcite and dolomite which could come from the marble

https://ydor.edeya.gr/labcheck/view?org_id=16609

I stayed in Paros for one week about ten years ago. The hotel owner (a local from Paros) told me not to drink tap water. I also saw locals buying large bottles of water at supermarkets.

According to testing of Paros’ tap water by both the Paros Public Health Department and the National Geological Institute— the water in Paros is safe to drink.

I also saw locals buying large bottles of water at supermarkets.

Just because they buy mineral water in supermarkets doesn't mean the tap water isn't drinkable. You might prefer mineral water because of the taste and its more natural quality.