Croatian water is among the safest in the world. They also make some pretty popular bottled ater too.
Despite this, it doesn't mean you are 100% guaranteed not to get diarrhoea. Traveller's diarrhoea is the most common complaint among tourists. All water has some microbes and usually harmless bacteria. Your body is accustomed to the mix in your home town supply, it gets a different combination in a new destination and may take a couple of days to adjust.
If diarrhoea lasts more than a week, see a doctor.
and check out our other safety tips for Croatia
phil from the safety hub. over 6 years ago