Nogbad,
Depending on the light, you may not need an ND filter at all. To show some movement in a water flow, you will probably be using speeds of between 1/15th and 1 second. The longer the speed, the more milky the water will appear. If you are using your slowest ISO setting and shooting at dawn/dusk, you may be able to get these sorts of shutter speeds without resorting to an ND. Shooting at these times may also give you more flattering light than harsh midday sun.
As for composing before or after, as you will be using a tripod, I'd compose before adding the filter as this will give you a clearer view in hte viewfinder; you are less likely to miss spotting a stray object in the scene, but I can't see that it will make a huge difference if you put the filter on first.
Regards,
Duncan
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