I don't recall Master Lin saying why yin-yang water is good for you.
So here's my take on it. In SFQ we strive for two objectives Balance and Flow. Our bodies are mostly water. We take water in. Water is a form of qi. This water will go to all parts of our body. Taking in water that is balanced, helps to balance our entire body.
... and if you drink enough yin-yang water you get lots of flow. 
You are perfection.
Iam2 
