These are two images taken from precisely the same point with the same camera at the same time on the same day.  The only difference between the two is that I have rotated the polarizing filter on the front of the lens through 90 degrees.  This has blocked the reflected light from the sky and allowed the camera to see to the bottom of the stream bed.  The band in the top picture is a stick on the far bank which has cut out the light from the sky so the bottom can be seen.  The sides have been swamped by the reflection.
Some people keep a skylight or other filter on the front of their lens to protect it from damage.  I have a polarizing filter on the front of mine.  This enables me to control the quality of the light and allows a stronger, more saturated, image and enhances the sky. 
When photographing water it gives me the chance to play around with the reflections to achieve the final result I want to achieve.  It does reduce the light by about half a stop but if it is that critical I can always remove it.