Buying the right equipment | Naturally, you are going to need to buy either a dedicated underwater camera or a waterproof housing to keep your existing camera dry. Before you do anything else, decide what it is you want to achieve. Are you hoping for some nice little pictures to show everyone when you get back home? Then a compact digital camera is a great choice – easy to use, light to carry and it won't require a lot of maintenance between dives. However, if you are hoping to blow your best ever picture up to poster size, you will need much higher calibre equipment and then you need to consider all that entails.
There are many good, up-to-date compact cameras that have waterproof housings and this is definitely the most cost effective route to take. You can also take some top-level SLR's underwater but this is quite a substantial investment so do your research into the market via a good, specialist retailer.
Buying a housing for a camera you already own and understand means you will feel more confident using it at depth and you will be saving on the initial investment. If you’re not sure what you want or even if you will like shooting in the long term, think about a second-hand set of equipment or even hiring. You can always trade up later on.
Lighting a photo | Next, and perhaps most importantly, take the time to gain at least a basic understanding of why taking a photo underwater is so different to above. Yes, there's a bit of science involved especially when it comes to understanding how light changes underwater: sunlight is filtered by water, so the deeper you go the darker it gets. In turn, this means colours fade because the deeper you go the less red light you – and your camera – will see resulting is blue, washed out pictures.
To return light and colour to an image you will need a light source. The built-in flashes in a compact camera aren’t strong enough to do more than light a nudibranch or small fish so you will need an additional flash or strobe. This will have to be mounted away from the camera lens otherwise you will get backscatter – reflections from particles suspended in the water. Even once you have done that, you need to be aware that flashes have their limitations too. They have to penetrate through the water, and the strength of their light reduces the further it has to reach.