A tricky question, as water moves at different speeds and can be in different light.
But let's start with bright sunlight and fast water. At ISO 100, the Sunny 16 rule says we're at f/16 and 1/100. That's clearly in the danger zone for water (neither slow enough to "flow it" nor fast enough to "stop it"). We'd really like to get to 1/3 second, maybe even slower, if we want to see the flow of the water. That's about five stops (100->50->25->12->6->3). So a 6-stop ND would give you what you seek. But if we're in shade with slower water, a two or three stop ND might be all you need.
Now for a trick: set you camera for multiple exposures (preferably not the wimpy 2- or 3-image ones cameras such as the Nikon consumer DSLRs have, but the 10-shot ones on the bigger cameras). Set Auto Gain. Make sure that your camera is absolutely on a rock solid tripod. Fire off the 10 exposures and let the camera stitch them into a single image. Since only the water is moving (hopefully), it's the only thing that's changing between the individual images, and the camera integrates those flowing water bits together into a single set of pixels at the end. Most of the time, this will work as a passable "ND" for water.
