It depends on the effect you want, but if you want the "silky" flowing water effect and you are shooting during the day when the light is bright you may need a neutral densiity filter.
On a bright say in sunlight your exposure at ISO 100 (the slowest speed on most consumer DSLRs) will be 1/100s at f16. To get the "flowing water" effect you'll probably need a shutter speed somewhere around 1/8s if you're failrly close, probably much longer if you're photographing a distant waterfall (several seconds might be needed). The shutter sped needed will depend on how fast the water is crossing through the frame. Without an ND filter you won't be able to get slow shutter speeds when the light is bright.
With a 3 stop ND filter (like the
Hoya 8x ND that 1/100s shutter speed will drop to around 1/13s and that puts you in the approximately right range for closup work. You can stop down to f22 and get 1/6s if you have to. Some digital cameras (like the EOS 5D an 1D series) have an ISO 50 speed which you can use to get a 1 stop longer shutter speed too.
If you shoot when it's cloudy or late in the day you might be able to get away without an ND filter, but even then an ND filter will give you a greater choice of shutter speeds and apertures.
You can use a polarizer as an ND filter, but it's only good for about 1.5-2 stops and in bright sunlight that's not really enough. Though stacking filters isn't recommended, you can also try stacking a polarizer with an 3 stop ND filter when the light is really bright or if you want shutter speeds closer to 1 second in bright light.
Don't forget the tripod, which you will certainly need at slow shutter speeds.
There's a small gallery of "flowing water" images at
http://www.bobatkins.com/photography/Gallery/flowing_water/index.html and I think they all have exposure information listed.
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