Shooting underwater is extremely challenging for many reasons. For one it is really hard to keep still and secondly your subjects are very rarely willing participants. But the biggest obstacle by far is having to deal with good lighting or the lack there of. Red and orange disappear quickly. By the time you get to 4 or 5 meters deep (15 ft) you can kiss red goodbye. At 7m orange is gone, at 12m bye bye yellow and you are left with greens and blues. At 20m green checks out.
It is also worth noting that light travelling horizontally through the water also loses colour. The light has to travel from the surface through the water to an object (fish) and then from the object to your lens.
In this awesome post-it illustration you can see that a diver 5m deep taking a photo of a fish that is 2m away has to factor in that the natural light will travel 7m.
Type of light underwater
There are two types of light to be concerned with:
Ambient light (ie: natural light)
Strobe or torch light (ie: introduced light)
Well actually 3 because in most cases unless you snorkelling on the surface there will be a combination of both that you need to factor in.
Natural light comes from the sun. You have the option to shoot three ways: into the light, with light behind you or with the light side on. Depending on the time of the day (and most organised dives tend to start early) you will have to factor this in.
You can use any of these ways to get great effects - cool silhouettes, contrast and shadows or
colour and detail, but used incorrectly they can spanner your pictures and videos.
Note the detail when the light source comes from behind you.
Playing with silhouettes. In the seahorse pic I was trying to introduce strobes so I could get detail and silhouette together with marginal success.
Some effects with shadow and contrast when the light comes from the side.
It is critical to anticipate the conditions and plan accordingly. Some days there will be a lot of sediment in the water and you are better off focusing on macro photography. Unless you live in the tropics where pretty much every shot will look better, the visibility is a big consideration in to how you set up your camera and what you should focus your efforts on.
You should aim to minimise repositioning your strobes while diving by having a rough idea about whether you are going to be taking reef shots, wide angle, macro, big subjects or small critters. You would not have your camera set up for macro if you plan to dive Manta Point. Conversely if you doing a muck dive at night you wouldn't have your camera set up to photograph a whale. Makes sense.
If you are on a budget a single strobe or dive torch can be used effectively and should in most cases be placed above your camera. Sometimes I will move the flex-arm to the side and focus slightly inwards to avoid backscatter.
Tip: when I started out my partner and I used our dive torches to illuminate subjects while the other one took photos. Getting your buddy to help you out is a great way for you both to learn about lighting and take better photos.
The white square on the camera is a diffuser. It covers the internal flash and helps spread the light to avoid backscatter.
A friend of mine cut out the side panel of a translucent white milk carton and fashioned it onto his housing.
In this photo, my girlfriend is shining her torch light on the weedy sea dragon, resulting in a great photo for her and blinding me in the process (but I'm grateful for the shot!)
There are many ways to angle your strobes depending on what you are shooting.
I commonly use the first two set-ups: the first for macro and the second for wide angle. The fourth set-up is great for when you are shooting on the sea floor and you don't want to over expose the bottom of the frame. The fifth set-up is good for caves or wrecks when you want the light to penetrate into the distance and give you depth. The last set-up is great for swim-throughs and narrow channels and allows you to take vertical shots.
Things to avoid
You want to keep your strobes facing forwards (not inwards) in almost all circumstances. The cone of light produced will ensure that the particles in the water in the area between your lens and the subject are not lit up.
Avoid kicking up sediment by frog finning and keeping your feet as far away from the bottom as possible.
Get closer to your subject
Use manual strobe power over TTL.
Try to get an even lighting spread across your images. Strobes angled in the wrong position or powered too high create a distracting and unnatural effect. Now depending on your level some of these photos might not look that bad and that is why I have included them. Once something gets pointed out we become a lot more aware of it.
Tips to avoid this: plan your pictures and power down the strobe that is closer to the subject (or power up the one that is further away).
Unfortunately I have many examples of hotspots.
This is a good time to talk about whether to set your strobes to manual power or TTL. TTL or through the lens is when your camera evaluates the scene and tells your strobes how much power is needed to expose the image optimally. Underwater it invariably gets this wrong. Sometimes it works and it realises that you focusing on a certain object and other times it tries to light up the background and it massively over exposes your subject and lights up every particle in the background.
My tip would be to always control the strobes manually and it is really not hard to do. Keep them mid to low when swimming around. Take a pic and then decide whether you need to power up or down. Once again anticipating your conditions and what you are planning to shoot helps. If I'm diving with sharks, they are likely to keep their distance so I'll power up to begin with.