When I was in Bali, Indonesia, I was not used to the idea that I couldn't just drink the water from the tap.  I started buying bottles of drinking water and quickly figured out that this was an expensive, wasteful and stressful activity.

There definately was a strategy to buying bottles of clean drinking water.  The bottles were much cheaper at markets compared to what was available at restaurants.  The price between markets was also very different depending on what type of market it was and where it was located.

My biggest revelation was that I could buy one of those large 5 gallon water jugs with an overall much cheaper price per liter of water.  I just had to buy a pump so the water could be pumped out easily.  The pump was only $3.

Once I bought the large water container and put it in my hotel room, I was no longer stressed about my access to clean drinking water each day.  I also no longer had to buy and waste lots of plastic bottles.  I just refilled my two large plastic bottles each day and set off on adventures. 

This might not be a good strategy if you are moving around a lot to different hotels, but if you are in the same place for three or more days, buy yourself a large 5 gallon water container and save some money and the environment.