Worked for me.
I used an IV line into one of the many vacuum hoses, and an intermittent "tap" at idle (which MAY have been pre-ignition) went away, and the car stopped "dieseling" run-on when switched off.
A spray would probably be better though.
Pouring from a cup is asking for a bent con rod.
To state the obvious, you need distilled, deionised water, or at the least, soft water.
Scottish tap water would probably be OK but its far too hard (probably artesian) here.
I used air conditioning condensate, which in the summer is available in huge quantities for free.