put your camera on a tripod, point it at fast moving, turbulent water. use shutter priority or manual modes to get shutter speed to a couple of seconds to start with, if in manual mode, change aperture and ISO accordingly. (You should be able to get decent results hand holding the camera, but nothing to this level.)
Shoot in raw in case you need to lighten or darken the pic (difficult to get good metering from reflections on water)
and just take some pics. change the shutter speed up and down just to see how it affects the final result. The smoothness you see is from the water never taking the same path. I don't have any pics to share to hand, or I'd let you know the settings I've used