Roman, I usually use an ND2 or ND4 filter on waterfalls depending on the weather. I've tried polarizing filters before on my old lens but was never able to produce what I wanted, where as the ND seemed to help more with my skill shortcomings as I was learning. I haven't tried with my new "waterfall lens" but I'll be picking one up in a week or two in preparation for my trip to Australia.

JP, I'll have to wait till I get home to send it out again, it might be a day or two.