In much of the world, tap water is unsafe. Unfortunately, bottled water is not necessarily better. Here is what you need to be on the lookout for if you have to drink bottled water.
People who live in advanced nations generally fail to appreciate the value of safe, germ-free drinking water straight from the tap. Although many Europeans eschew the taste of tap water and insist on mineral waters bottled from natural springs, the fact is, it's OK from a health standpoint to drink water staight from the tap most of the time in the United States, in Canada, in Australia and New Zealand, in the European Union, in many parts of the Caribbean, in Korea, and in Japan.
Even in developed countries, however, broken pipes and flooded water treatment plants can make tap water temporarily unsafe to drink.
Boil Water Notices
In the United States, municipalities and municipal utility districts that run water supply systems keep daily check on the microbial content of tap water. From time to time, water gets contaminated when pipes break, when pumps fail, or when heavy rainfall causes sewers to overflow. When this happens, local television and radio stations will broadcast "boil water notices," if the problem is bacterial and can be fixed by simply boiling water, or warnings not to use the water at all in rare cases of chemical contamination.
If you are staying at a hotel, the management will be aware of this problem, and will probably make sure you have bottled water. Typically, boil water notices only last as long as it takes for a specific problem to get repaired, such as replacing a broken pipeline or a failed pump. Similar warning systems exist in the UK and Ireland, Australia and New Zealand, and Canada.
If you get an infectious disease from tap water in developed countries, you almost have to have tried.
Montezuma Extracts Revenge Far Beyond Mexico
On the other hand, there are some countries where drinking the tap water is almost guaranteed to cause problems. So many visitors from the USA get E coli infections from drinking water in Mexico that the condition has come to be known as "Montezuma's revenge." (As a personal note, I have visited Mexico over 40 times and I have never had a problem while I was in Mexico, but I always get symptoms when I return to the States.)
In other parts of Latin America you are probably fine drinking water from the tap in your hotel, but you may run into problems when you stay in rustic accommodations.
St Petersburg in Russia has a notorious problem with the microorganism Giardia , the causative agent of a condition that causes intense diarrhea and a kind of flatulence in reverse known as "purple burps". This microbe also is found in clear mountain streams in the northwestern USA and in the Himalayas. Generally speaking, any place that isn't wealthy enough for sewage treatment plants doesn't have safe drinking water, either.
Problems With Drinking Water Aren't Limited To The Water You Drink
Travelers who assume that a little swish of contaminated water is acceptable make a serious mistake. Using bacterially infested waters to rinse out your mouth after brushing your teeth, or enjoying an ice cold beverage poured over ice made with contaminated water, is also a way to get infected. Rinsing fruits and vegetables to remove contamination doesn't work when the water used to rinse them is filled with microorganisms. So what can a traveler do?