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Most of the issues have been discussed by other answers. However I would like to clarify something about *pure water* (a.k.a. "distilled water" or "demineralized water"), which is "apparently" not a good conductor and would seem to pose less danger than tap water or other "dirty water".

To avoid giving a false sense of security, I will state loud and clear that **also pure water is dangerous** in a "water spill + electricity + human body" context. The reason is twofold:

+ **Impurities coming from the environment.**
+ **Chemical reactions triggered by current.**

Let's examine those points in more detail.

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**Impurities coming from the environment**: As someone already said in this thread, water with added contaminants (dirt, salts, dust or whatever) is fairly more conductive than pure water. As a consequence, **when pure water is spilled, it is very easily contaminated by the impurities it finds in the environment** (unless the spill happens in a "hyperpure" environment) and so it becomes much more conductive.

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**Chemical reactions triggered by current**: this second problem is more subtle, in fact pure water in the presence of electricity can "create" its own impurities! Especially when DC currents are involved, pure water can combine with the materials it touches and start corrosion processes and other chemical reactions that can can lead to ions from metals or other substances to migrate into the water itself, making it highly conductive. 

It's like a chain reaction sometimes: pure water makes a copper wire corrode; copper ions enter the water making it more conductive; more conductive water makes the current bigger; bigger current enhances the corrosion; still more ions enter the water: rinse and repeat and you have no longer "pure water", but a highly conductive water-based electrolyte that is perfect to make contact with human skin or cause havoc in a circuit (you *can* have short circuits due to electrolyte spills)!