Outdoors in the sun, yes.
Indoors it doesn't do much at all (although it can cut reflections off a window).
At night you'll be losing too much light (but then, you can enhance/remove light-reflections off the water, and you'll probably want the light-loss to make longer exposures to blur the water).
And when it's cloudy, it depends on how lazy i'm feeling, it doesn't do much either way.