a water injection installation is a last line of defence type of solution against detonation, a engine should be running proper a/f ratio's to start with. obviously the rest goes without saying, proper maintenence, a properly built motor to start with (proper bearing clearances essential)
with normal boost and a well built engine with correct a/f ratio's you should never run into detonation, although the stock ecu's do have knock detection, so i guess even nissan thought it might happen sometimes.
with that said i myself run a aquamist install on my nissan, exactly for peace of mind.. just make sure whoever installs it knows their stuff..