Waterfalls can be very satisfying to photograph, and are great subject for landscape photography but they are not easy to do well. Very bright days almost always lead to average pictures. Much better to go with subtle light to achieve pleasing images. My technique of photographing waterfalls has always been to shoot on very overcast days - this works well as it reduces the contrast in the scene and avoids too much 'blow-out' where the whites of the water rapids occur.
These days I adopt a slightly different approach where possible, and that is to use a subtle variety of light. With the correct conditions you can shoot the surroundings of the waterfall in slightly brighter light, whilst the waterfall remains more in shadow. The effect only needs to be slight, but it can make a real difference. Variety of light on a scene will almost always get you better landscape photos: in the case of waterfalls it can work perfectly, as the lighter (sometimes almost pure white) waterfall remains more in shadow and will be better exposed. Think of shooting a landscape with an area of snow in the shade - you end up with variation in lighting but the snow still shows up well because it reflects more light even though its in shadow.
The two images below were all taken using this principle. In this case, at the cul-de-sac of Cotter Force in the Yorkshire Dales. The weak sun was gradually coming round over my right shoulder, so the autumn colours on the left were beginning to catch the light whilst the waterfall remained in the shadows.
When getting up close to the water I think it less important for a variety of light, and I would generally prefer to use purely overcast conditions (or shade) to reduce the contrast and get a more even exposure.
Waterfalls are often included on my landscape photography courses in the Yorkshire Dales. Working out in the field is the best way to learn in these situations and see how the light really affects the whole scene.
