From the How Does He Do That File.

I get tons of compliments on my flowing water shots either on Facebook, various photography discussion forums and Flickr over the years, capturing the right image depends on some decent equipment and some artistic judgement. To get the silky water texture you see here you need either a nice film camera with 100 ISO film (colour) or 50 ISO if you are shooting black and white. Now if you have a DSLR most cameras will have a base ISO of 100. You will also need a decent quality tripod, either a Manfrotto, Giottos or Vanguard and they start around $300 with a  decent head.  The cheapie ghetto specials from Blacks, Costco or Future Shop are not going to cut it, the camera has to be super still. Finally you will need Neutral density filters, they can either be the screw in variety you can install on the front of your lens or the more expensive sheets that fit into a holder.

The whole idea is you want to control the amount to light hitting either the film or sensor plane at the same you decent sharpness across your depth of field while keeping the shutter open for longer than 1/15 of a second. Some photographers want to keep their camera shutters open for longer than a second, I find you lose definition in the water in that case. My sweet spot is between 1/8 to 1/2 a second in terms of exposure you get some texture in the flowing water. The camera itself is affixed to your tripod so there is no shake while you take the picture. Even in this day and age with image stabilization, still pays to use a tripod.