We have more than one boat equipped with water injection, and there are others in the field. The watered pipe makes the pipe tuning adjustable. If the pipe were just longer all the time, it wouldn't produce the extreme RPM needed to get well into the over-rev. The pipe effect has to be longer to get the hulls moving at lower speeds ( on big SAW blades ), and shorter at higher speeds to pull big numbers.
Our system regulates the water injection for when it's needed, and when it's not. Driver controlled.