The most common approach to taking great flowing water pictures is to use a long exposure. This allows the "soft, dreamy flow" of water to be captured as you have probably seen in many photos. Achieving a long exposure may require extra equipment, depending on how the scene is lit.

Long Exposures

To achieve a long exposure, you will need to reduce the amount of light reaching your sensor, and expose the sensor for a long duration of time. This usually requires a very tight aperture, such as f/11 or higher, and a long exposure, anywhere from 1/6th of a second to several seconds. You should always use the lowest ISO setting your camera is capable of, as this helps in extending your exposure times. Normally this is ISO100, however if your camera goes to ISO50 or lower, use the lowest setting you can achieve.

In addition to your camera exposure settings, you will need to use a tripod to stabilize the shot. Lengthy exposures in the large fractional seconds to full seconds can not be stabilized by hand, even with the most powerful IS/VR. A tripod will not only allow you to stabilize your shot, but they often have levels that help you keep your shot horizontally level.

To further minimize camera shake, a couple other tools can help. If your camera supports a Mirror Lockup feature, use it. This allows you to flip the mirror with the first press of the shutter, and take the shot with the second. Sadly, pressing the shutter often causes greater camera shake than the mirror flip itself, so an important tool is a remote or cable shutter release. These provide a remote controlled or cable-connected shutter button that allow you to flip the mirror and release the shutter without touching the camera, eliminating camera shake.

Tuning for the Scene

Which settings you use will be dependent upon the scene. Brightly lit scenes may require a very tight aperture. I would avoid using an aperture tighter than f/22, as around there your image will start showing noticeable distortion due to diffraction. I would recommend keeping your aperture between f/11 and f/16, and if you need a longer exposure, add an ND (neutral density) filter to the front of your camera. An ND filter will reduce the amount of light that enters your camera, allowing you to expose for a longer duration.

The longer the shutter is open, the more it will capture that soft sense of motion in water. The amount of motion you wish to capture can greatly affect your scene, so the longest exposure time is not always the most appropriate. A "shorter" long exposure, anywhere from 1/6th of a second up to perhaps 1 second, will show the motion and shape of water better. A "longer" long exposure, up to several seconds, will smooth out the shape of water and enhance that "soft, dreamy glow" effect. The longer you expose, the glassier flat, clear parts of the waters surface will get.

Dimming and Deepening

I highly recommend investing in some ND filters if you wish to do long-exposure shots of water. Even if a scene does not explicitly need an ND filter, they help to mitigate the volume of light and give you more control. ND filters can help deepen the contrast and vibrance of a scene, and can add to the overall effect.

Alternative Scenes

Beyond just photographing waterfalls, longer exposures can create a nicer effect for a wide variety of water scenes. From simple streams or rivers, to lakes lakes, to expansive ocean coastline shots, or tighter shots of reefs and tidal pool areas around coast lines. Longer exposures of water can smooth out the noisy detail of ripples and waves on the surface, reducing the complexity of a scene, helping users zero in on the true subject you are trying to portray.