photoman022 wrote:
The reason you'll need a longer shutter speed (and a tripod--even though you don't want to use one) is that an ND filter is dark; it holds back light; you'll need to compensate that loss of light by either increasing your ISO, f/stop, or shutter speed. To get the silky effect of water you need to increase the shutter speed, which will make any camera shake very apparent if you hand hold the camera.
Sorry, but this comment is very misleading to the typical beginning photographer, which the OP appears to be. The main purpose of the ND filter (for the condition the OP asked) is to slow down the shutter speed. Using the f stop or ISO to compensate for the loss of available light would just defeat the purpose of using it.