Sunglasses for Cameras?

The waterfall is one of the most interesting landscape subjects to photograph. Eric and I both like to see the water blurred instead of frozen in place. The shutter speed needed to accomplish this varies depending upon the speed and volume of the water. The shutter speed should be slow enough to blur the water but fast enough that the water does not loose all detail. We use ISO 100 and an aperture value up to F16. Sometimes the resulting shutter speed is still too fast and we need to reduce the light entering the lens. We use neutral density filters that act like sunglasses to get to the correct shutter speed. It is generally a trial and error method but by bracketing the exposures, we find a combination that produces the result we desired.