It will be tough for it to work like you describe, EFI cars don't have a constant vacuum port like a carbureted car does.
When you go WOT, when you will need the water the most, there are no ports with vacuum to draw the water in. Carbs have venturis, and vac ports in/near the venturis that will have vacuum even at WOT.
You may want to rig in a WOT switch and run a high pressure injection pump & nozzle that activates after ~70% throttle