When visiting a foreign country, water may be far down your list of things you want to think about. You'll probably be too busy drawing up a list of sights to see and things to do, and water won't even figure in the equation. However, depending on the country you are visiting, this could be a very grave mistake.
In the Western world, we are used to being able to drink water straight from the tap. It's one of the products of Western civilization, and we do it without even thinking. However, certain countries - such as those in the Middle or Far East - do not have quite the same sanitary system as Western countries, and the tap water is unsafe to drink.
First and foremost, this means you will need to purchase supplies of bottled water as soon as you arrive. Do not underestimate the amount of water you need; it is better to have too much than too little.
Secondly - and most forgotten - you need to think about the times you ingest tap water without thinking. For example: brushing your teeth. When in one of these countries, replace running the toothbrush under the tap with dunking it in a glass of bottled water.
Finally, don't think this is a rule that you can avoid. Quite frequently the tap water in these countries will look perfectly safe to drink; fresh, clean and clear. However, if the country has a no tap water advisory, stick to it. If you don't, you could be in for a ruined holiday due to stomach upsets and illnesses. It really is better to be safe than sorry.