Is safe to drink tap water in cafes and restaurants, or should I only drink bottled water?
If you have an "iron constitution" you could probably get away with it, but it's not recommended.
canidrinkthewater.org is a great resource I always check with, and they say "no", stick to bottled water.
Traveller's Diarrhoea is one of the most common ailments - no matter how good the local water might be.
All water contains microbes and 'stuff' (that's a medical term) particular to that location, your tummy is used to the concoction in your location, and when you move you get a different combination of microbes which might cause upset and it takes a day or two to get acclimatised.
Here're some tips on TD and dealing with it, and how to recognise it's become a health risk.
Phil. almost 9 years ago