I've never quite understood the need for ND filters when shooting waterfalls. Probably 50% of the shots I take are waterfalls, and I've yet to use an ND filter on any of them. Granted, I'm usually f16-f20 depending on the scene and almost always ISO 50 and sometimes ISO 100. It only takes 1-1.5 sec. of exposure time to get a nice blue action and sometimes that is too much depending on the amount of water flow. Still, there are times when I expose for as much as 15 seconds depending on the scene.
1. I ALWAYS shoot with a CPL, whether I'm using it to it's full effect on the scene or not... typically always have some dialed in however.
2. Maybe the falls are just different here, being typically in a heavily wooded area without much light even in mid day.
I have used soft-edge GND's at times, but only when part of the scene is significantly brighter than the rest resulting in too much range.
I know people use them, but personally I fight the exposure times being too long as opposed to not long enough.