Most places that deal with tourists in developing countries usually clearly have marked on the menu that they use purified water even for washing salad items so you should be fine but if in doubt, ask for the drink with no ice.
The WHO has done a big push throughout the developing world about sanitation and most locals in these countries also boil their water before consumption - it makes them sick too. In Cambodia last year, there were pictorial signs everywhere from World Vision promoting boiling water for drinking and washing hands.
However, water may upset you as can a change of diet, just because it is different or richer from what you are used to. To be safe, especially if you have a sensitive stomach, make sure you only consume bottled drinks.
I've been to Peru, Cambodia, Viet Nam, Indonesia, Egypt, Tonga, Ecuador, Thailand and many other developing countries and have only been sick once - it was the mayonnaise on a cheese sandwich that got me. You'd think that a cheese sandwich would be "safe"
I had to laugh at one guy on our Cambodia trip who was totally paranoid about "getting sick" - he only drank drinks without ice, but the ones he chose were made with coconut milk, he ate coconut milk curries for lunch and dinner, for breakfast he ate large quantities of fruit and porridge - he got an upset stomach very quickly - but purely due to sudden intake of fibre combined with the high fat content in the coconut milk. He though he was doing the right thing but actually if he'd just drank water and stirfry type dishes he probably would have been fine.