Fuel injector for water injection?

[tgentry]

I'm sure I'm overlooking something because this seems way to simple and elegant:

Why couldn't we use a fuel injector for water injection? Use the same signal the ECU sends to the fuel injectors for the water injector. That way fuel and water would always be at the same ratio. Tune the water/fuel ratio with a pressure regulator or by changing injector size. Add a hobbs switch to turn it on at the pressure that suits you.

Ok let me have it, why won't this work?