What is the "boundary water" in Feng Shui?

Boundary water means that Yin Qi will stop when it meets water in coastal areas, rivers and lakes.

The sun be the spirit met a gap and moved away.

This is the characteristic of yin and yang.

Yin Qi drops, Yang Qi rises, the water at the border of Yin Qi stops, and Yang Qi runs when it meets the air.