I recently went to a Tom Murphy seminar where he recommended an ND filter to slow the shutter to 1 second when shooting water. I probably went a little overboard with the 3.0 filter and if you want to try this I'd suggest something a little less extreme, maybe 1.8 or 2.0. But this is fun stuff and I love what it does with the image.
The B/W conversion is custom, starting with the LR low contrast preset. I added an LR graduated filter to darken the rocks in the water at the bottom of the frame - kind of the opposite of it's typical use.
Nikon D3, Nikkor 18-35mm @ 18mm, 2.5 seconds @ f/8, GPS