Shui Qi (or Water Curve) is found on the Gallbladder channel on the dorsal aspect of the foot. You will find this point 1.5 cun proximal to the web margin of the 4th and 5th toes. Another way to express this, is that it is 1 cun proximal to Liu Wan (Sixth Finish - 66.08).
Check out eLotus' excellent video tutorial on locating and needling this point.