Waterfalls are one of my favorite subjects to photograph.
I typically like to use a slow shutter speed to capture
the motion of the water. This can be problematic during
bright lighting conditions, as the situation will typically
call for a fast shutter speed. To ameliorate this, I use
a polarizing filter, along with a number of neutral density
filters, to reduce the amount of light passing through
the lens, allowing the use of a slower shutter speed.
A polarizing filter also helps to reduce glare and reflections
from the water surface, wet leaves, rocks, etc. A sturdy
tripod and remote shutter release are especially important
to ensure a sharp photograph, free of any camera shake.