For such a simple question is not always able to answer correctly, and the reader, I hope, will not complain to us if we explain briefly, what is this action of water on the fire.
First, touching the burning object, it is converted into steam, taking a lot of heat at the burning of the body; to turn scalding water into steam, you need five times-some more warmth than to heat the same amount of cold water to 100 degrees.
Secondly, the pair formed at the same time, occupy the volume hundreds of times greater than that gave them water, surrounding a burning body, a pair of push the air, and no air combustion is impossible.
To increase ognegasitelem force of the water, sometimes add to it... gunpowder! This may seem strange, but this is quite reasonable: the gunpowder quickly burned, releasing a large number of non-combustible gases that surround a burning objects, hinder combustion.