Water injection,
alone does nothing to raise power. An engine will lose power if running at the same rpm and load when water is injected, boosted or not.
This was determined way back in the 1930's and they came to the conclusion that to allow higher resistance to detonation in boosted (supercharged) engines distilled water and methanol were mixed together 60/40 and subsequently as 50/50.
Mixing with methanol had the effect of lessening the amount of power lost which also brought cylinder head temps down to allow higher boost pressures than could be tolerated in the same engines without WM50. It's known as Anti Detonation Fluid, -ADI nowadays, because that is what it does. It's not a power booster in itself.
The benefits are that it keeps the combustion chamber very clean as there will be no carbon build-up. It also allows engines to run on really low octane fuel without suffering knock and can help with economy & emissions.
The other consequences are that water is, as everyone knows, very corrosive especially to engine components. It also kills O2 sensors quite quickly. The amount of extra maintenance required to carry out corrosion prevention regularly in a normal production engine is not worth the hassle frankly.
http://www.snowperformance.net/
Here is a brilliant link that says it all...........
http://www.rbracing-rsr.com/waterinjection.html
Reading around this website
proves Americans have a sense of humour (esp. see tab "eat sh*t") here:
http://www.rbracing-rsr.com/