Why isn't water flammable? ?

Water is combusted hydrogen. Combustion or "burning" is reacting with oxygen. So water is the product of combusted hydrogen and can't by burned any further. The same if you burn carbon, you get CO2 or carbon dioxide. CO2 (like water) is used to extinguish fires because CO2 can't be further oxidized (reacted with oxygen). Remember oxygen is not flammable, it only makes things burn better.