Natural water #(H_2O)# is not pure, and contains many dissolved ions in it. The ions are free to move, and that's why usually water is a great conductor of heat and electricity. So, by putting water on something hot, the water will absorb all the heat and evaporate, leaving all the least energetic molecules on the hot object. This cools the object down. Not to mention, water will unlikely do any harmful reactions, unless you put it on an alkali metal (which you shouldn't try!).
That's why water is often used as a coolant.