Water is "already burnt" H. That's what water is. When you "burn" H, the result is H2O. Oxygen or O2 does not "burn". Another word for burning is "oxidizing". That means combining something with oxygen. Another way of looking at it is that the oxygen extracts electrons from the other thing to burn it. So to burn water--or oxidize it--you'd have to find something that could extract electrons from it. Problem is: the electrons in water are perfectly and beautifully satisfied and cannot be extracted easily.

However, water *can* be oxidized very slightly and the result is H2O2 or hydrogen peroxide, but a chemist would not call it "burning". "Burning is a trivial term," said one chemist, prefering the term "oxidizing".

One of the most critical events in the development of the earth, around 3 billion years ago, was the emergence of bacterial organisms that could oxidize water. Before this, there was little oxygen on earth. These bacteria evolved biological molecules--a complex of proteins--that worked together to remove electrons from water to provide energy for the bacteria. Oxygen was a byproduct.