Shutter Speed Choice: The water flow will be fast with a large waterfall and slow with little stream, so the shutter speed will vary for each scene. In addition, depending on the focal length of the lens, it will provide less motion (wide angle) or more motion (telephoto).
I photographed the waterfall in Iceland while leading a photography tour in the horizontal image has a .6 second shutter speed for a satiny effect, while the vertical has a 1/10 shutter speed to get more detail.
In another example, the ocean images have a 10 second exposure to create the smooth look of the ocean surf.
In Yosemite, I use around 1/125 of a second for having some detail in the powerful, fast moving waterfalls yet 1/15 to 1/30 of a second for smooth streams in the valley.
Tripod: Using a STURDY tripod will be necessary for the slow shutter speeds. It is still a good idea for higher shutter speeds, as it will help with fine-tuning the final composition. Keep in mind it is often windy at the base of a waterfall or around the ocean surf.
Exposure: when taking a photograph, I decide whether the shutter speed or f/stop is the most important and set that first. Normally, I use manual mode and set the shutter speed then set the f/stop I want for the scene when photographing moving body of water as that is the most important. Next I would set ISO. I would use the native ISO for the camera, such as ISO 100 for Canon, or 200 for Nikon. If the shutter speed is too slow, I would raise the ISO to the proper exposure. Finally, I would use a filter, as discussed below.
Shutter Speed: In order to get slow shutter speeds for the satiny effect, try photographing in low light conditions as full sun maybe to too strong for slow motion. For example: photograph at low light near sunrise or sunset on sunny days, with the subject it by the first or last rays of light or when the water is in the shade. That can be on an overcast day or after sunset or before sunrise.
Filters: Using a polarizer will reduce your shutter speed time by about two f-stops. Turn the polarizer to see the effect on shinny rock surfaces. Be careful when using a polarizer so as not to take out desired colorful reflections. Neutral density filters, grey in color, will reduce the light to the sensor, allowing for a slower shutter speed.
Ideas: Some ideas include waterfalls, streams, cascades, ocean surf or a lake in stormy weather. I like photographing streams in the shade with green leaves reflected onto streams in the afternoon (Yosemite’s Fern Spring is good for that). I like to photograph along Yosemite’s Merced River at sunrise to get the warm reflections of the mountains in the river. Fall colors are another good one to look for.