These three filters will help you make better images.
Polarizing filter. Benefits: Reduces reflections on water (and glass and foliage), but it can also make your pictures look sharper by reducing reflections on atmospheric haze.
Variable ND filter. Benefits: Lets you shoot at slow shutter speeds in bright light to create the "beautiful flowing water" effect.
Topaz Labs filters. Actually, Topaz offers many plug-in filter sets for creative and corrective image making. Topaz Black & White Effects is just one.
Speaking of three: These 3 books will make you a better photographer: http://bit.ly/1rHIrEi
Explore the light,