You'd run water injection on boost when charge air temps are above a certain threshold. You need a tank of water to inject, then you need to consider when you'd need it. All the time? How much water do you need to carry? How much flow?

It's a real specialist way of cooling the intake air that you can generally do with a bigger intercooler. If your intercooler size is limited and there's no other way, then it may be worth doing.

Charlie