Scouting the viewpoint with a cutout!
Dan,
The way to take pictures like this is with a tripod and cable (or time-delayed shutter release). The slow shutter speed does indeed give the milky appearance that you desire. The rest of the picture can be sharper. Still, even before this, the choice of the approach to the scene is the hardest part. As a start, (and perhaps you do this already), I recommend having a cardboard cutout with a rectangle corresponding to 4x5, the form of prints. It can be 8mm x 20mm and then you look through this as you move from position to discover a viewpoint you find most interesting. This means you can move around without your camera. I trained 3 sons with this method, using, instead, fingers to make such a viewing rectangle. As they crouched climbed up to a new vantage point or came up from an new angle, the arrangement of things changed and so they discovered the form of things. Such effort, I believe s the most important part of learning photography.
The next part s simply taking a myriad of pictures, each time choosing the best and destroying the rest. Then simply print out these few in B&W and draw on them with a wax marker notes for yourself as to what you might want to improve.
When you can make a picture as you had hoped, then you are ready for experimenting with all the great suggestions Fahim has shared.
In this case, I'd venture to take his idea of using B&W further. Why not only use B&W presentation for the next 50 pictures! This will allow you to clinch, shape, form, light and shading, gesture and the interplay of the components of your picture. So, starting with this picture of a waterfall, can we look at it in B&W? I did this within moments of it being posted.
What software do you have for this, if that's what you might like to do? Trust me that simplifying your framework for your photography will help you make better pictures. So you want to try this. We don't teach here but if you are sufficiently motivated, we'll share our experience wth you.
Asher