The water is perfectly fine to drink although it's always a good idea to carry a bottle around just to rehydrate! Keep a few bottles in your hotel room as well! On the isles, I always use bottled water..but will drink the water when offered at cafes, etc.. EnjoY!
Why is Santorini tap water salty?
But because the drinking water is ocean water that has passed through desalination plants prior to reaching your tap, many people report that the water tastes brackish and salty. In fact, this is probably why so many places in Santorini's hospitality industry offer water bottles instead.