Photos that feature milky-smooth flowing water seem to have a Marmite effect on people: they're either loved or hated. I'm often rather ambivalent towards them, but it doesn't mean that it isn't a useful technique to have up your sleeve if you're faced with a weir or waterfall and you want to capture an image with smooth-looking water that has a sense of flow to it.
There's no great secret to shooting a photo that has water flowing through it that looks smooth: it's done using a long exposure. The slow shutter speed captures the the water as it moves, making it blurred. The blur, in this instance, gives the water a smooth appearance.
Shooting long exposures in daylight hours comes with an inherent problem, however. Over-exposure. Our cameras' sensors are capable of detecting far more light than we think they are, and even using the lowest possible ISO and smallest available aperture, a long exposure can result in an over-exposed photo when taking during the day. To get around this irritation, you might want to try a neutral density (ND) filter over your lens.
ND filters are grey filters that cut down the amount of light that enters your lens without affecting the colour of your images. They come in different grades, or densities, blocking out between one stop and 12 stops of light. Screw one over your lens and you'll give yourself a great deal more flexibility when it comes to shooting daytime long exposures.
Then of course you'll need a tripod. You might want to capture the motion blur of the water, but you'll want to avoid camera-shake and the rest of the scene getting the wobbles. Even though you'll be using a very small aperture with an enormous depth-of-field, still think carefully about your framing of the shot and its point-of-focus. Make sure it's telling a story.
Obviously you'll need to have your camera in manual mode to ensure that you can adjust the shutter speed, ISO, and aperture to get the photo that you want. Almost certainly you will need to use the lowest ISO and smallest aperture avalable. When it comes to shutter speed, you might find that you need to venture into bulb mode to get the shutter speed you need. And we recommend that you use a remote shutter release to prevent jolting your camera on its tripod and shifting its focus, too.
Then it's a case of hitting the cable release and leaving the camera to do its thing.