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That silky smooth water effect is all about exposure and understanding how light will effect the exposure, be it digital or film. To get it right takes in many factors including the ambient light, stability of the shot, and tools to lengthen exposure as need be.
But first a primer on exposure, and how it applies to this type of shot. Each point, be it digital, or film, experiences exposure due to light. The exposure is cumulative, and a one way trip from dark to light. Every time a bright point hits the plane, that point is that much brighter, and will NEVER get darker.
This is where is gets fun. Water is chaotic, and depending on volume, will create a chaotic number of bright points over a period of time to create the cloud-like illusion. The amount of chaos is a function of the volume of the water, and the disturbance in it's path. A slow moving stream can take 8 seconds, a river at flood stage 1/5 sec.
The question begs, how to get 8 seconds on a sunny afternoon? Well, likely you don't. You can use neutral density filters to help, or at times, a circular polarizer to limit the reflections you don't want. But your best bet is late afternoon or early in the morning (I've heard of such a thing), or overcast, or best, while it's raining. Yes, while it's raining.. the water droplets from the rain give a nice tone, and make the scene more chaotic, often lessening the needed exposure.
Tools I use: Camera with exposure control, solid tripod, remote shutter, circular polarizer, and a nice scene with the right light. Optional are the ND filters JIC the light is to much.