Shui Xian, also called Water Sprite, is on the Kidney channel 2 cun below Kidney 3 or Shui Xiang. This point is often used with Shui Jing and Shui Xiang.
To locate this point, find the Shui Xiang point (aka Kidney 3) and measure 2 cun distal to it. This point is on the calcaneal bone.