nicolesy provides a good overview on getting started with long exposure water photography:

Basically, to create the misty, cotton-candy water effect, you need a very long exposure. Sometimes the light is too bright for the camera to leave the shutter open long enough, so you need to block the light that is coming through the lens so that you can prolong the exposure time and use a slower shutter speed

Good advice on technique, equipment and apps to assist in getting good results.