Shooting a flowing stream or a waterfall with an exposure time of a second or more will give you white water from the blurred movement. This gives a smoothed appearance to the water, showing the movement in an unusual way.
But to create this effect successfully you will need to prepare. Firstly you should use a tripod, or at the very least somewhere to rest your camera as you are aiming for a slow shutter speed. You should then select the lowest ISO setting you can on your EOS.
Next select Aperture-priority AE (Av) mode on your EOS and select the smallest aperture (largest f/ number) available. Together, these two actions should give you a slow enough shutter speed to provide the effect. If not try using filters such as a Neutral density filter or Polariser which will reduce the amount of light entering the camera.
Try different aperture settings to see the effect that this has on shutter speed and the final image.
Alternatively, try slow shutter speeds when you are shooting in a rain shower. Each individual raindrop will travel a short distance during the exposure, giving elongated streaks of water. The best effects are obtained by shooting into the light, so that the raindrops are lit from behind. Again, do remember to use a tripod or stable location to rest your camera.