How does the water appear as smooth as silk?
The secret is long exposure.
In this photo I exposed 160 seconds with F16 (aperture):
A large F number (small aperture) will give you a larger depth of field which is perfect for scenic photos. And since we have to get more lights entering into the camera, we need to have long exposures, say 160 seconds.
The longer the exposure time, the smoother the water will be.