I like to expose moving water using a shutter speed of about a half second to get the silky flowing motion. The problem it creates is that the water in the pool has the same effect, and I don't like that particularly. So I expose with a slow shutter speed and a fast shutter speed and composite the two. That way I get the motion I want in the cascade but the stop action I like in the pool.
Photo by Allen Utzig
Cascade RiverLake SuperiorNorth ShoreMinnesota