Waterfalls can be tricky due to wind at times. If there is a significant volume of water moving, then typically all of the bushes/trees nearby will have movement. With longer exposures, this creates blur issues with the photo. One way to combat this is by using a technique referred to as exposure blending. All this means is you are taking two or more photos. One with a slow shutter speed, and the other(s) with a faster shutter speed. The idea is to capture your water movement in one photo and freeze the frame in the other.
While in Oregon, we visited Dry Creek Falls, and I highly recommend the four-mile hike if you visit the Cascade Locks area. It is two miles in and a little over a 700-foot elevation. Sounds tough, but it was easy. The walk through the woods cannot be beaten. On a rainy day, you will be surrounded by ferns, and other plants with water drops on them helping to create endless photo opportunities.
In most situations, you do not want the elements at the front of the photo blurry or out of focus due to that being the first thing most viewers see. The plant in the bottom left of the frame was the anchor for my shot, but you can see several parts of it are out of focus due to movement. This photo was taken at 1/6" of a second. So now that I had my shot that had the water movement that I liked, it was time to take a faster shutter speed photo to freeze the plants. Both shots were focused on the plant, so do not worry about redoing focus if you are trying to learn the technique. The other essential part to remember is to adjust your exposure. If you merely change the shutter speed, you will have an under/overexposed photo. Using your meter will help you figure this out.
For this shot, I used a 1/30th of a second shutter speed. Once I got home, I wish I had used a faster one, but oh well. It still did the job even though you can see some slight movement in leaves around the middle of the image. To process the files, you will need to open them in Photoshop as layers. There are many ways you can blend images, but for the sake of simplicity just create a mask on the faster shutter speed image. Select the brush tool, select black as the color, and paint over the water to bring the water flow from the other photo into view. Once you have the image as you want, create a composite of that image by pressing control+shift+option+E on Windows, and on a Mac, I believe it is Command+Option+Shift+E. Once you have your composite, you can begin to process the file like you normally would a single image. I would keep the layers just in case you find something down the road you do not like.
My final image.