I did a day's outing last summer with Experience Seminars, Canon pro's here in the UK. To sum up their advice:

Overcast conditions better than bright sunshine (to prevent burn-out on the water)
Large waterfalls usually work better with high shutter speeds e 1/250th and above, to freeze action - but with very large waterfalls you may need to use speeds of 1/1000 or more to freeze the water.
Smaller waterfalls usually work better with shower speeds eg 1.15th sec and below to give soft effect
(But take photos of the waterfall reversing the above and see which effect you like)
If working with slower shutter speeds and with a tripod, use a spot or partial reading off a mid-tone area and then set up camera accordingly
Consider taking small sections as well as whole waterfall.
Keep an eye out for rainbows and use a polariser to intensify colours
Wide angle lens eg 16-35 mm lens for whole waterfall, or longer lens for smaller sections
Small aperture (i.e. big number) eg. f/22 for depth of field
Up the ISO since waterfalls are usually surrounded by trees and foliage
Tripod and cable release
Polariser filter
Lens hood (to keep lens dry in drifting mist)
Take lens cleaning cloths in case mist still gets on lens
Take care how you change lenses, if you do, because of the mist/spray/damp.

I set my camera to manual focus, focus and then use Live View and magnify 5 or 10 times to make sure I am in focus.

I attach a photo of a waterfall in Yorkshire taken this last summer following the above advice. Settings were:

Aperture Priority
f/29
ISO 100
Lens 24-105mm set at 65mm
2.5 seconds
Tripod/Cable release

Hope some of this might help you.

Hi lightchime. The info from Canon is guidelines, not gospel, and you always have to tailor the basic info to the circumstances, don't you.

It wasn't a sunny day, it was quite overcast. I didn't need to up the ISO because there were not a lot of trees around, the waterfall is quite bright anyway and I thought that with a higher ISO the water would burn out (as it already has on the edges as it tips over the rocks). I didn't use the recommended very wide-angle lens since I wanted the whole waterfall and I could not get very close, so using my wide angle lens would have made the waterfall very small. I took it both with a fast shutter speed and a slower one as recommended, but preferred the image with the softer look of the water. I used the small aperture (f/29) which has given good d-o-f. As you see, I used my tripod and cable release for a 2.5 second exposure, and I certainly used manual focus and Live View to get it as sharp as it is on the rocks. I didn't deviate much from Canon's guidelines, and that day at Ingleton was my first attempt at waterfalls. This picture won best of the evening for a landscape comp at our camera club. Not that I am saying it is marvellous, but it pleases me (and fortunately, also that particular judge!)