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Not to be picky.....but at what ISO and f/stop are you shooting?
You have a lot of variables you are dealing with here, and all of them are equally important.
Let's deal with a couple of them.
1. Light. Shooting waterfalls is ALWAYS best done under cloud cover heavy enough to eliminate shadows, or at first/last llight when the sun has gone off both the water and the immediately surrounding landscape that will be in the picture. This is an issue of dynamic range, where the sunlight off the water is so much brighter than the surrounding landscape that the camera simply cannot capture both light and dark with any detail. You simply have to manage this situation with your shooting plan or travel plan.
2. Reflection. Always a scene killer on moving water, it adds to the dynamic range problem. A good circular polarizer will help here. You want one that is LARGER by at least one size than the filter ring diameter of the largest lens you will be using. I use an 82 mm and have a set of inexpensive step down rings that let me use it on lenses down to 52mm. This avoids the corner darkening that you will get by using a filter sized to your lens. Most polarizers will cut the light by at least 2-4 f/stops when in use, which might be enough to get you where you want to be if you are using native ISO (100 for Canon, 200 for Nikon) and an f/stop beyond f/11-16. If you do nothing else, get one of these....preferably the most expensive you can afford. Quality costs money here.
3. Cutting the light. See Tareq's note above. Using a neutral density (ND for solid color across the entire filter, ND Grad for dark at one side and totally clear by the other side) filter is the classic solution here to give you a longer exposure. NOTE....it will not do ANYTHING for management of extreme dynamic range if you are in a sunlight water situation. Check out the options at B&H or Adorama. Cokin P holders are the least inexpensive, but you will have to buy a set of adapter rings for your various lenses. Lee are made better but more expensive. BOTH WILL darken the corners of any photo shot with lenses greater than about 70mm. An alternative many people take is to not buy a holder at all, and to buy the larger size filters (for medium format camera lenses) and then hand-hold them in front of the lens. Doesn't work for me, though. Anyway, A decent initial kit of these would be ND filters in 2, 4, 6 or 3, 5, 7 stop strengths, and a set of ND Grads in the same strengths. Be very careful handling them. The ones you and I can afford are acrylic plastic and scratch quite easily. If you visit the Singh Ray site you can find some very nice andi nformative videos describing these filters in detail and showing them in action in the field. I HIGHLY RECOMMEND watching the entire series of videos before you buy any filters.