Water is something that's so obvious, and so vital, and - for most of us in the West, at least - so commonplace, that it's something we take for granted. It's just there: when we want it, we help ourselves. Considering that even in the most favorable circumstances you will die after three days without water, it's sometimes a little strange what a lackadaisical attitude we have to the stuff.
If you're a traveler you must never take water for granted. Even if you visiting a relatively temperate western country with a moderate climate and potable mains drinking water, it's vitally important to remember to drink enough. Traveling is a very dehydrating business, not least because you spend so much time in planes, trains, hotels, shops and museums, where the airconditioning can dry you out very quickly simply by removing the moisture from the air you breathe. Additionally, if you're the kind of vacationer who likes to walk around a lot you wind up taking a lot of exercise - you don't notice, because you get no ore out of breath than usual. But you use up a lot more of your body's resources tramping around a foreign city than you do in a day at the office, and water is the most valuable and important of them.
If you're traveling or having a reasonably active vacation, and especially if you're visiting a hot country, you should aim to drink at least four or five liters more than you usually would on a equivalent day at home. Your kids, too, should increase their input. Luckily, it's very easy to find safe supplies of water in most tourist destinations - to be on the safe side, stick to major brands of mineral water. As you probably already know, drinking a little, often, is the best way to stay hydrated. Don't forget to eat, either, as water is no good without the salts and nutrients your body needs to accompany it. If you're dehydrated a small, salty snack or a banana (rich in potassium) can really help to make you feel better fast.
One of the problems of drinking more, of course, is that you have to visit the bathroom more often. This often puts people off drinking as much as they should in strange cities, as they don't want to get "caught short", as the English say. One of the best bits of vacation planning you can do is research in advance where you can find public restrooms. Museums, theaters, restaurants and large public buildings are usually the best places to look!