Since you have the benefit of immediate review, do this.
Unless you want a "shakey" image, these types of images are taken with the camera on a tripod.
Set your camera to shutter priority.
Depending on the flow of the water and the effect you want, set the shutter speed to 1/4 second, then 1/2 second, then 1 second, then 2 seconds. One of these shutter speeds should give you what you want.
I think for daytime even with NDx8, you still need to shoot at f22, or the smallest possible aperture to maximise the shutter speeds. Depending on weather, you may be getting shutter speeds from 2-8s. Of course, the longer the better if you want silker water, but there's a limit in the day, unless you want to resort to stacking more filters.