Water is the life source of all living things, but when it's contaminated with parasites or bacteria then it becomes a killer. If you're staying in a high-class hotel, then bottled water is going to be available to you on request, but in some of the smaller boutique hotels and hostels, it's water on tap unless you brought your own. Drinking water that isn't bottled is highly risky, and at the very least will probably give you a stomach ache. It's something to avoid doing at all costs.