Start with a Polarizer. That could slow things down 2 to 2 1/2 stops. I just came back from a Workshop and that's all we needed. Don't shoot it in bright sunlight. The water will be blown out. We spent early morning shooting the waterfalls, and moved on to Wildflowers in the shade once the sun was up. The water in the sunlight will be so hot, you'll lose all detail. We were also bracketing five shots for HDRs. We really walked away with some really nice shots.
It depends on many things, but anywhere from about two stops to about eight stops. Two stops assumes that you're not under open sky, and that you're willing to stop down to f/22 or so. Eight stops happens if you're NOT willing to stop down that far, or if ambient is pretty bright, or if there are bright highlights in the water that you don't want to blow out. It also depends on just how much blur you want, and how fast the water is moving. You need longer exposures to blur things like water lapping at boats in a harbor. Conversely if you have fast-moving water you may well be able to tolerate quite a few stops more shutter speed.
I only have about ten stops worth of filters, and that may be constraining my experience. Writing this I can easily imagine using another five or six stops under some conditions.
_____ Brian... a bicoastal Nikonian and Team Member
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It depends on the intensity of the blur you are after. The strength of the ambient light is also a consideration. Generally, in order to get any kind of blur or "veiling" as it is sometimes called, you need at a minimum about a 3 second exposure. If you are looking to get that smoky, dreamlike quality you need at least 30 seconds and closer to a minute or more. This is especially true when shooting ocean tides since you need time for the waves to roll in and out. At a minimum a 3 stop ND filter will work and I use as much as 10 stops of filtration.
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As stated above, it depends on the ambient light level and the effect you want to achieve. I would start with a minimum of a three stop ND filter and go up from there. if your budget is robust, you might want to consider a Singh-Ray Vari-ND filter that varies between 2 and 8 stops. You can then combine it with a 3 or 5 stop ND to get 11 or 13 stops which should be enough for most if not all situations.