By telling them the truth?

Tapwater in Iceland is safe to drink. Went there a couple of years ago and spoke to the locals regarding bottled water. They told me that the bottled water is the ‘same’ water I get from the tap.

In France I once went to a water museum, yes those exist. They told us that tapwater is safe to drink and that we should stop buying bottled water.

I live in The Netherlands myself and I don’t know better then drinken water from the tap. I would go as far as saying that we are among the countries with the best tapwater in the world.