I've been shooting waterfalls for 45 years and I live in a waterfall rich area. (Near the Columbia River Gorge.) I shot them obsessively until I switched to shooting pretty women.
The waterfalls around here are mostly in canyons that never get good light on a sunny day. ND filters don't help, because you have too much contrast between the white falling water and the dark rocks and trees.
I simply wait for a cloudy day or I shoot early in the morning before the clouds burn off. If I am travelling some distance to photograph a particular waterfall, I make sure I understand it's geographic orientation. Sometimes with that info, I can make an educated guess as to when there will be good light on the falls, like just before sunset, for example. Shoshone Falls near Twin Falls Idaho is a good example of a waterfall that faces West for a nice sunset look. It took me two visits to figure that out! Sometimes if you wait until evening when there is no direct sun, you can get the dim low-contrast light that you want.
The key thing to remember is that every waterfall is different.
If you are travelling and only have a brief opportunity to shoot a particular waterfall, you are often screwed by the nasty bright, contrasty light. If you are visiting for the first time, you probably won't know what lens is best, so bring all your wide angle lenses. It might be helpful to study photos of the particular waterfall on the net to see what light and what shooting positions others have found attractive.
If you happen to get a good cloudy day you are golden. Set your shutter speed at 1/10, your ISO at 50 and you should have a small enough aperture to do the job. You won't need a filter unless you want to turn the water into fog, but that's up to you. Don't be afraid to chimp the LCD screen to get what you want.
That's my method and I'm sticking to it!