Mastering the breathtaking wide-angle scenes that you see in NatGeo images is the most difficult type of underwater photography to get right. It requires a lot of planning, forethought and framing as well as knowing the limitations of your equipment and working with that accordingly.
Firstly colour is lost at depth, both vertically (ie: the deeper you are) and horizontally (ie: the greater the distance between yourself and your subject).
Secondly wide-angle photography it is much more complex than macro photography because there is more to consider when composing your picture: the main subject, elements in the the background and the foreground, the colour of the water and the potential for backscatter created by using artificial light.
To take great wide angle scenes you ideally need good visibility with minimal particles in the water. Tropical holiday destinations often offer 30m visibility and clear blue water. My local conditions in Sydney are often in the 5-10m range and make taking wide-angle pictures much more challenging. But it is still achievable as shown in the photo below.
Before we get into this topic make sure you have a grip of this basics in this article on underwater lighting.
Subject selection
This section really should be titled background selection because it is the most important. Get all the elements of background selection correct before thinking about your subject or waiting for it to swim into position.
Notice how in the first image the kelp distracts the viewer from the sea dragons nose
If your scene is a colourful reef think wisely and consider the many elements at play. You want contract, colour, rich blues and clarity of subject. Resist the urge to just point and shoot because from your point of view the reef looks amazing. The chances are your field of vision is wider than your camera's field of view and your perception has been altered by all that you have seen in the lead up to taking your photo.
Pay attention to the foreground and use it wisely to create a sense of depth. Have your subject correctly in focus. Make sure that elements in the background don't create unwanted noise or distract attention away from your subject.
As always remember that not every scene or subject will make a good photo Sometimes it is preferable to just admire your environment than try to capture it. I have often had someone excitedly point out something awesome, but I can see that taking a great picture of it is impossible given the background or my camera's capability. I sometimes take an obligatory photo of it to appease them knowing full well that I'll delete it later.
Below are two examples of pictures with distracting backgrounds and too many elements in the photo that do not contribute positively.
Think about what you want to shoot before you press the shutter trigger. If the animal is moving quickly ie: dolphin or seal you will need a faster shutter speed so that the animal is clearly in focus.
Shutter speed will effect the background, making it either darker or lighter. So set a shutter speed in advance that gives you the best "blue" background while allowing the subject to remain in focus.
Lighting using your internal flash/diffuser
If you are using a compact camera with an internal flash/diffuser you need to get very close to your main subject in order to illuminate it, bring out its true colours and minimise backscatter. Anything further than 2ft away will not be illuminated correctly.
There are steps you can take to minimise this. Shoot in good light at a shallow depth (5- 10m) to allow ambient light to bring in some colour. You can later edit your pictures and bring back some of the colour that is lacking. If you take a photo beyond 15m without strobes you will have no colour to bring back in post-production editing.
This picture was taken in very shallow water with a waterproof compact.
When shooting at depth be aware that your photos will only contain blues and blacks and hence think carefully about your subject. This can be used to great effect to showcase a gloomy, eerie shipwreck or you could consider making the image a black and white one.
To demonstrate what is possible with an affordable camera setup I selected the two images (below) that I took using a compact camera: the Canon G16 and only using the internal flash - no strobes, no fancy gear.
In this first image of a manta ray, I was at a depth of 9m (29ft). There was enough ambient light due to the fact that it was sunny, midday in shallow and remarkably clear water.
This second image was shot at a depth of about 15m (50ft) but I was close enough for the internal flash to have an effect and it brought out some of the colour on the shark.
Lightning using strobes
When using strobes or artificial light you need to be aware of the how far the light will carry and which elements will contain colour and which elements (ie: the ones further away) will have a blue cast. Using strobes will allow you to illuminate within up to 1.5m (5ft) of your main subject or foreground. Beyond that you will be working with blue tones.
Position your strobes as wide as possible to allow even lighting of the foreground and/or your subject while minimising backscatter.
I would suggest setting your shutter speed in advance to achieve the correct background shade of blue and then use your strobes to correctly expose your subject and foreground.
Using ambient light to your advantage
Here are some considerations:
Consider shooting scenes from below your subject with the camera facing slightly upwards to take advantage of ambient light
Time of day determines the angle that the light will be coming from
Shooting with the light behind you to get deep rich blues and illuminated subjects
Shooting into the light for silhouettes (see pic below)
Framing
Pay attention to the corners of your image on the LCD screen. This can be hard because you are so focussed on the shark and making sure that it is in the frame that the other elements escape your attention. Look at what adds to your photo and look for what distracts. Are there elements that crowd the image or don't create sufficient contrast to your main subject? Does the reef behind the shark make it stand out less? If so, position yourself and snap the image when it moves into the blue.
Depth of field
In order to get as much of your image in focus you will need to close your aperture (ie: higher F-stop number). Remember that this will make your photo darker and you will need to decrease your shutter speed and increase your ISO to get the correct exposure. You ideally want to adjust all of these settings by a small amount rather than one of them to a greater amount.
I set my ISO at 100 is shallow, clear water and adjust it higher to 400 or even 800 as I get deeper or the water gets darker.
Final tips and mental check list
Get as close to your subject without bothering or chasing it away
Consider the background and use your shutter speed to set the correct shade of blue
Set your strobes as wide as possible - pointing them forwards, not in or out
Sometimes it is preferable to swim away from the scene and get the settings right and then return, take up position and take the photo
Don't overexpose the foreground with your strobes - it will look unnatural