Underwater photography is one area where you definitely need special equipment, and special techniques to go with them. 

    There are two main solutions to the problem of taking a camera underwater:  dedicated underwater cameras, and housings which keep regular above water cameras from getting wet.
    For some years most professional underwater photographers used dedicated underwater cameras as their primary means of doing underwater photography.   Although several systems exist, including cameras by companies like Sea and Sea  with their Motor Marine line of cameras, the market was dominated by a division of Nikon called Nikonos, and by one camera in particular, the Nikonos V.

   Nowadays, it seems as if the dedicated underwater camera market has split into two separate markets, one with a bright future and the other with a rapidly approaching dead end.   Although the Nikonos V was the favored camera of professional underwater photographers from the time it was introduced in 1984, the following years saw a drift away in other directions.   In particular, the advent of auto-focus cameras in housings allowed people to take photographs which were simply out of reach of the Nikonos.   This was particularly apparent in the area of macro photography, where the Nikonos approach was particularly cumbersome and ineffective.   Nikon responded with its $US8000 Nikonos RS, a very attractive dedicated auto-focus underwater camera released in 1992.   This produced photos every bit as good as housed cameras, and the inherent smallness of dedicated cameras compared to housed cameras was also a signficant advantage, allowing easier handling of the camera underwater.   However, the market for this camera was too small, it was plagued with flooding problems and in 1996 production ceased.   Then in September of 2001 Nikon announced that they were also discontinuing production of the Nikonos V.
    The other market has a much brighter future: disposable one-time-use underwater cameras for completely amateur holiday snapshots.   Don't be deceived - these cameras are strictly low-end affairs, often with no flash, or only a feeble on-camera flash with all of the problems that implies.   Someone who wants more than just poor quality reminders of what they saw will soon be disappointed with these cameras, but they're extremely cheap and maintenance free, making them a good entry point for someone who isn't sure whether or not they want to dive deeper into this hobby (ha, ha).   Although they are very limited, the economic reality is that these cameras will be around as long as film survives, and they might actually be a force when most other film photography has vanished.

     Most housed cameras are placed into rigid boxes made of thick aluminum or clear polycarbonate plastic, both of which will survive deeper dives than most recreational divers will ever encounter.    There is also a third option manufactured by a company called EWA Marine, consisting of a thick plastic bag with a glass "port" that the lens fits into.   The controls are manipulated through the plastic, and some of the housings even allow you to use a regular flash unit mounted in the camera's hot-shoe!   However, the deeper you go the greater the water pressure becomes, with the result that the plastic can start to press against the camera itself, making it difficult to use its controls.   This is a good solution for situations above water where damage from water or dust are a serious danger, but most underwater photographers will find these bags too limiting.

    The rigid housings have advantages over dedicated cameras, such as the ability to see water entering before the camera is damaged (a particular benefit of the clear housings), and the ability to use the housed camera for general use out of the water, but they also have disadvantages.   For starters, they're relatively bulky and heavy, which is a particular hassle if you're travelling by air.   The housings are also specific to particular cameras, since the external buttons and knobs for controlling the camera have to be placed in exactly the right location for particular models of camera.   Therefore, if you've got more than one type of camera body, you'll either have to carry several housings, or restrict your underwater work to one of the bodies.   As you can imagine, there are also issues associated with fitting large telephoto and wide-angle lenses into the housing.   For reasons which will be explained shortly, nobody uses telephoto lenses underwater, so you don't need to worry about that, but the same reasons which count against telephotos also mean that wide-angle lenses are particularly valued underwater, and the "ports" to allow them to be used are bulky, fragile and expensive.

    Another issue with housings is condensation.   The camera will heat up as you use it in this enclosed space, and the difference in temperature between the cool water outside and the warm air inside will result in condensation collecting on the inside of the housing.   I'm not sure what the situation is with aluminum housings, but with the clear polycarbonate housing I use, this condensation always seems to be worst on the glass plate that the lens is supposed to shoot through!   The condensation can be dealt with by reducing the amount of water suspended in the air within the housing.   In the steamy tropics where most underwater photography is done this is a particular challenge, but opening the camera only in the dry air-conditioned atmosphere of a hotel room can help, and small sachets of silica gel placed in the housing itself can eliminate the problem entirely.   The silica gel should be shut in the housing some hours before you dive, and you should leave it in the camera when you dive.   Another way to reduce this condensation is to reduce the amount of heat the camera produces, by turning off or reducing usage of its LCD(s), and even by allowing the camera to go into "sleep" mode if it isn't used for a minute or two.

    Underwater photography is significantly different to terrestrial photography.   To begin with, the underwater world is, and will always remain, a foreign environment to animals like ourselves.   If you're not alert, you can go from happy to dead in a shockingly short period of time.    Below the surface, the continued maintenance of things we normally take completely for granted, like the air we breathe, is a major technical hurdle.   No-one is exempt - scuba divers face the dangers of the bends and nitrogen narcosis, but even snorkellers can suffer from the little known condition called "shallow water blackout", which is much more likely to hit a camera-equipped snorkeller than other snorkellers, since photographers are more likely to over-exert themselves and so run out of oxygen.
    Apart from these entirely man-made dangers, you also need to remember that this is a dogfish eat dogfish world, and a surprising number of creatures here are protected by poison, including the coral reef itself, not to mention crinoids and beguilingly attractive critters like cone shells, which are small and very beautiful but have enough punch to kill anyone ignorant enough to pick them up.   The rule, then is this - don't touch!   If you don't touch anything, then you won't harm it and it won't harm you.

    In addition to these potentially lethal problems, there are other matters which are merely troublesome rather than a matter of life and death.  In particular, this is a far more three dimensional environment than the one we're used to, where our feet are normally planted firmly on the ground.   Moving up and down within the water column takes a lot more effort than we expend in our normal two-dimensional lives, and for photographers the techniques required to maintain a motionless position long enough to get photos can be surprisingly tricky.   Scuba divers wearing buoyant wetsuits also need to wear heavy weight belts, and they have to use buoyancy compensation devices (BCDs), which are inflatable and deflatable jackets which can be used to fine-tune whether they float or sink.   I've never scuba dived, but when I'm snorkelling I also wear a weight belt in order to achieve neutral buoyancy.   Since the pressure at different depths compresses the air in my lungs to different amounts, I actually float when I'm near the surface, and sink when I'm 25 or 30 feet below the surface.
    Another aspect of photographing in a three-dimensional environment is learning which camera angles work and which don't.   It shouldn't be too much of a surprise that photographs of retreating fish are unattractive but so, often, are photographs of fish taken from above.   It's hit or miss whether a fish photographed from below will result in a good shot.
 

    This photo illustrates a few of the points I've made so far.   Here I am photographing a Napoleon Wrasse on the Great Barrier Reef using a Canon G2 digital point and shoot camera in an Ikelite housing and with an Ikelite flash.   The guy photographing me is using the very same camera and housing, but he has a wide-angle converter on the lens to allow him to get closer to the fish and me, and he also has a more powerful Ikelite flash unit than I have.   Despite these two advantages, and the fact that we're in relatively shallow water on a very sunny day, the colors in this photo are not natural.   The swim suit I was wearing was bright red, but it appears almost black even though I've used PhotoShop's "adjust color balance" feature to tone down the blue and intensify the yellow and red.   Scuba divers who cut themselves underwater experience a similar unnerving sensation - their blood appears to be black because all of the red light has been absorbed!   Incidentally, in this photo you can also see the weight belt I wear when snorkelling underwater to give me neutral buoyancy.

    There are two solutions to the light absorption conundrum: restrict all of your photography to sunny days and the top ten feet of water, or use a powerful flash to restore the light and color.   If you've only got an underwater point and shoot camera without an external flash, then you've got no choice except to take the first piece of advice; otherwise, you need to use an external flash, which underwater photographers most often refer to as a "strobe".    Even an external flash isn't a cure-all solution.   Water not only selectively absorbs light, it absorbs light, period.   As you can see from this photo, even the most powerful strobe has a range of only around 10 or 12 feet, far less than a flash used in air.

    Another serious problem with using strobes is called "backscatter".   It's extremely rare to have truly crystal clear water; instead, there's a certain number of suspended particles present even in water which seems totally pure, another example of the human brain filtering out things which it doesn't consider important.   These particles often aren't a problem if you're doing available light photography, but as soon as you start using a strobe it becomes a much more serious issue.   If you don't think you'll be using a strobe much, then you better go back a couple of paragraphs and read again what I said about light absorption!
    The best way to think about backscatter is to imagine the particles as small spheres made of shiny, silvery metal.   When the light from the strobe hits these spheres, the light is reflected back towards the camera, resulting in a photograph that looks like it might have been taken in an underwater snowstorm!   There are two main solutions to this problem: shoot your subjects from close up so there are fewer particles between you and them, and move the strobe onto an arm which is a significant distance from the camera itself - doing this will reduce the area of the particles which is strongly lit, resulting in less backscatter.   You can see a typical strobe arm in the photo above.   The other piece of advice, about shooting from close up, also fits in well with the short range of all underwater strobes.

Macro Photography

    If you're determined to master the field of underwater telephoto photography then you'll have a uniquely lonely road to tread, one without fellow travellers and without any equipment, either.   In short, there's no such field, because of the problems of light absorption and backscatter which have already been discussed.   There is, however, a considerable field devoted to wide angle photography, which is particularly suited to underwater work, because wide-angle lenses have great depth of field and allow very close focussing.   When combined, these two features allow you to place a large foreground item in the frame at a very small distance from the camera while making items in the middle distance appear to be quite far away - resulting in a shot which exaggerates the clarity of the water, which is a nearly magical feat of great value in this type of photography!
    Another area of underwater photography which maximizes the advantages of short camera-to-subject distances is macro photography.  Again, backscatter is minimized and the short distances allow small apertures to be used.   Whether above water or below, macro photography turns small critters which might be easily ignored into fascinating explorations of a miniature world whose shapes, patterns and colors often exceed the beauty of the larger worlds around it.
    Something else to consider is that the refraction of light through water means that any lens which is taken underwater will have a focal length multiplication effect applied to it - an underwater lens with focal length of 35mm will have about the same angle of coverage as a 50mm lens above water.   This is a true focal length multiplication effect, completely unrelated to the "field of view crop" experienced by digital cameras whose CCD or CMOS sensor is smaller than the size of a 35mm film frame.   If you do take such a camera underwater, then not only will you get the focal length multiplication caused by refraction, you'll also get the field of view crop, turning a 35mm terrestrial lens into an 80mm equivalent lens underwater, assuming that you're using a camera like the Canon D60 which has a 1.6 times field of view crop factor.
 

Digital Underwater Photography

    If you're doing digital underwater photography then you're doing housed underwater photography, since there are no dedicated digital underwater cameras.   Several manufacturers make housings out of different materials for different cameras, and as is usual for housed systems, there's a different housing for each different model of camera, because the positions of the camera controls tend to vary considerably even for different models from the same manufacturer.

    Digital underwater photography has a number of benefits over film underwater photography, quite apart from the absence of the usual hassles of film photography at airport x-ray security checkpoints and the processing, development and cataloguing of negatives.
    An advantage provided by point-and-shoot digital cameras used underwater is the ease of use provided by the large LCD at the back of the camera.   Compared to squinting through the viewfinder of a housed digital or film SLR camera or even the viewfinder of a dedicated underwater camera, the LCD makes accurate framing of each shot a breeze, and also provides instant feedback about whether the exposure was good or not.   Being able to photograph creatures from an arm's length also allows you to get shots of skittish animals which would otherwise be impossible.
    One of the nicest things is the elimination of the need to open the housing to load a new roll of film.   High capacity compact flash cards can store far more images than a roll of film could ever hope to do, so it's possible to keep the housing closed for one or even more entire days of diving, considerably reducing the risk of flooding caused by unseating of the o-ring or accidental introduction of foreign bodies between the o-ring and its seat.   You also won't have to worry about losing that shot of a lifetime which materializes the moment after you've shot the last frame on a roll of film; nor will you need to push your limits of time and oxygen trying to find interesting subjects to fill those final few unused frames on the roll, either.
    On the flip side, you might find that you're not able to make full use of those high capacity cards, because digital cameras use far more electricity than film cameras, and there's a good chance that your battery will run out of juice long before the card is full.   Running the battery flat is mostly an issue if you're free diving rather than scuba diving, since a freediver can remain in the water much longer than a scuba diver.   Digital point-and-shoot cameras, which use the LCD to frame the shot, are more prone to this problem than digital SLRs, because the LCDs on the back of the camera chew up a lot of electricity.   You can save electricity by telling the camera to automatically power down after a minute or so, or disable the LCD entirely and learn to "shoot from the hip" by setting the zoom to wide-angle and then pointing the camera in the general direction of your subject before pressing the shutter release.   This might sound unworkable, but with just a little bit of experience it's a practical solution to this problem.

    Despite all of these advantages, at the time of writing near the start of 2003, digital underwater photography still hasn't reached a level which will satisfy high-end professional underwater photographers, most of whom are accustomed to shooting with very fine-grained slide film around ASA 25 and ASA 50.   Four megapixel point and shoot cameras and six megapixel SLRs like the Canon D60 have housings available for them, but although this is enough for even very advanced amateurs and those who are displaying their work solely on the internet, it's not considered adequate by professionals and many magazine photo departments.   There can be little doubt, however, that digital cameras will soon dominate even the high-end market, though hold-outs will doubtless be fighting an increasingly desperate rear-guard action for some years to come!