A fire needs three things: oxygen, a type of gasoline, and a heat source. Water (H2O) and water having more hydrogen than oxygen because you can't breath under water. The water takes away the oxygen. I hope I didn't over or under explain it.
A fire needs three things to perpetuate itself:
- fuel
- oxygen
- a net return on heat energy to foster the further combustion of fuel.
The source of oxygen is the air which is 1/5 oxygen. Water is hydrogen already oxidized so there is no more energy to be had from having it oxidize further. It can therefore act as a barrier between the fuel and its supply of airborne oxygen. This is not true of any metal that is capable of stealing the oxygen from the water eg. all alkaline metals, some alkaline earth metals, and especially Aluminum.
The most fire retarding property of water however is its ability to absorb copious amounts of heat. Water has a very high heat capacity and will steal all heat from a fire so there is no return on heat energy to perpetuate the fire.