Why does water put fire out?

Very little although the word corrosion can imply things other than oxidation.

It does mean that the reaction between hydrogen and fluorine is… interesting.
http://io9.gizmodo.com/an-explosion-that-shows-why-fluorine-is-the-tiger-of-c-1516351149

But if elements are highly reactive, then their post-reaction compounds as a broad generalization will tend to be more stable. Fluoride for your teeth is a complicated story, but the stability of the incorporated fluoride mineral is essential.

Maybe life is how fire found a great new way to survive. Living things are all about collecting and using energy after all. We’re all burning in a sense.

The Master speaks:

When fire evolves a resistance to water, we’re all in trouble.