The bright light off the water has caused your camera to underexpose so the picture is dark and muddy. Were you working with the camera on auto?
You need to take an exposure reading off a darker area of the sky, e.g. the sunset area itself, if you can. If not, try taking a reading off the palm of your hand and using that exposure. It helps, too, if you can set the camera manually.
With digital, you can experiment with different exposure settings if you're quick enough to get the best.