A few years ago I went camping up in the White Mountains of New Hampshire with the intent of capturing some waterfalls. Unfortunately, the sun was out and despite stopping down to f/22 I was unable to expose long enough to get the wispy strands of water I was looking for.
A month later I tried again, with greater success, because I brought with me 2 ‘secrets’ of a good waterfall shot: a neutral density filter and a polarizer. A neutral density filter simply reduces the amount of light coming in, making the scene ‘darker’, and enabling a longer exposure. The polarizer, in simplest terms, blocks out reflected light. The polarizer is so wonderful and versatile, that I no longer go anywhere without it. It makes waterfalls more stunning, and skies bluer. This link shows a couple of nice examples (not mine) of the polarizer at work.
For waterfalls, the polarizer removes the reflection of light off the wet rocks, which can overwhelm the photo otherwise. Here are three images showing the same scene with the polarizer at maximum (left), intermediate (middle) and minimum (right) effect. Amazing difference!
The waterfalls below come from at least 2, maybe 3 different trips to the White Mountains.
Arethusa Falls 
Arethusa Falls 
Arethusa Falls 
