I do quite a bit of work with water and long exposures so here are a few ideas and a little critique for this shot. (others may have different opinions or methods)
Because of the nature of the shot (long exposure) a tripod is essential as is a circular polariser. The CP should be rotated in such a way that it reduces any reflections from the light on the water which will result in a nice shot of the stones under the water. A wide angle lens will result in the opportunity for foreground interest. Shooting in bright sunlight can result in over exposure on the rushing water on which the light is shining. A low viewpoint is always an advantage as it allows you to get that foreground interest in that I mentioned.
Looking at your shot, you've got that bright sunlight I mentioned, this can be compensated for in post production a little. It looks like you have shot looking down onto the subject...maybe had you got down even lower (and probably got wet and dirty) you may have had a more pleasing shot. I find it's always quite difficult getting the result "just right" when working with small waterflows like this. Maybe my technbiques are better suited to larger waterfalls.
Hope that helps a little, have a look at some of my shots using long exposures either in my portfolio on EPZ or on my website: www.colouredimages.com
, in particular the waterfalls section