WHAT? and WHY?
Water Injection is the process of injecting water (or a water/alcohol mix) into your engine. It is introduced in a very fine mist into the air flow upstream of the intake manifold. Especially on forced induction (turbocharged/supercharged) cars, this process is designed to abosorb heat that is created when that air is compressed. This creates lower Intake Air Temperatures, which can also result in running advanced timing, having lower EGTs, etc.
Note: This process is not to be confused with Intercooler Misting which is the process of spraying the exterior of your intercooler(s) with water.

THE SYSTEM
I chose the Aquamist 2d system. The Aquamist 1s system uses only a single pressure switch to determine when to inject water, and thus it is merely on or off. Thus, the same volume of water is being injected regardless of RPM, or more specifically regardless of the amount of Air or Fuel the car is consuming. In the Aquamist 2d system, there is also a pressure sensor, so you can control a limit that the system will not operate below. Once you are above that set point, this system reads your fuel injector duty cycle and injects water at a fixed ratio to your fuel. This method can ensure that you don't inject too much water at lower RPMs or too little at higher RPMs. With either system, you can still change the size of the injection jet, which will scale the amount of water injected.

SYSTEM DESCRIPTION
See the first photo below.  The left side of the diagram begins the system with a water tank (I chose to use my washer fluid reservoir). Aquamist supplies a threaded tubing connector for this connection. Water flows through the 6mm tubing to the next item, the Water Filter. After that, it connects to the input of the pump. The outlet side of the pump has 4mm tubing which goes to a Water Pressure Switch. This switch tells the pump when it needs to operate in order to maintain the desired water pressure in the line. After this, more 4mm tubing connects to the High Speed Solenoid Valve. It is this valve that controls when water is injected into the system. This valve is controlled by the Fuel Injection Amplifier circuit which uses the Fuel Injector and the Boost Pressure Switch to determine when to inject water (as described above). After this valve, tubing connects to a Jet. In this section, I have inserted a Splitter to provide water to both halves of the bi-pipe. There is also a Yellow LED included which connects to alert of a blocked water jet (or other flow obstruction).  The green LED flashes to the pulse of the fuel injectors and the Red LED illuminates when water is being injected.  I have also added a switch to turn the system off when desired.