Working professionally as an Underwater Photographer, I dive everyday with my camera. Positioning my camera at upwards angle has become somewhat natural to me. But why is it important to shoot up?
Well as any diver should know the deeper under the water you go the darker it gets. The main light source, the sun, is always above you. You may notice your shadow on the sand below you when you are in clear shallow water. Now imagine you see a nice featherstar on top of a rock. What will happen when you put your camera directly over it? A shadow is created, right over the starfish.
This is a problem because shadows automatically reduce the light and therefore the brightness of your picture. As the whole subject is covered in shadow you will not get any definition, which is usually created by the contrast between the lighter and darker areas. This definition adds depth to the image, giving the subject a fuller shape. So a picture taken downwards, in shadow will look flat.
Now take that same featherstar and move the camera to the side, at a 90-degree angle, already you have taken away the shadow and allowed the sunlight to naturally create brighter and darker areas. The photo now has some definition and the subject will stand out more.
But you might find parts of your image are still a little dark. Lets try moving the camera below the featherstar, so the lens is at a 45-degree angle towards the surface. This allows more of the sunlight to enter the lens, therefore brightening the image.
The following three images illustrate the different angles. It is the same Featherstar in each picture. The pictures were taken one after the other, during a mid-day dive. I have not done any editing on the photos.
You still need to be aware of where the sun is. Just as it moves across the sky above water, you will see its position change throughout the day underwater as well. This will affect what type of picture you can take. The lower the sun the longer the shadows. Try to shoot with your back to the sun to get the optimum light on the subject, but at the beginning or the end of the day pay attention to where your shadow is falling, you may need to move so that the sun is slightly to your side.
Another option is to shoot silhouettes. With large subjects you can easily place the sun behind them, or just above to get a sun burst effect. You can also shoot towards the sun but leave it out of the frame. The front of your subject will be dark but something in the background may be lit up.
Adding a second light source such as a strobe or two can create nice effects, especially with silhouettes. The extra light can illuminate the front of the subject, leaving the middle ground dark with a sun burst in the background. Strobes can also be used to lighten very dark shadows that may appear when the sun is to your side or directly above.