The silky smooth water effect is a bit overdone these days - maybe try a double exposure with a long exposure to smooth it and a fast one to freeze it then play in PS (or similar) to see the different effects.
If you're after long exposures, try to avoid using the smallest apertures (highest numbers), stick to around f/11 or so to avoid softness due to diffraction. Use extra ND filtration if necessary.
To get an adjustable ND effect, try using 2 polarising filters - if using "modern" cameras (AF, AE etc), the one fitted to the lens will need to be a Circular Polariser. The one furthest from the lens will need to be a Linear Polariser. By adjusting the relationship between the 2, you'll be able to adjust how much light gets through as well as the usual polarising effect.