Underwater photography is a style of photography that can be fun and rewarding as well as unworldly. There are certainly things one can do underwater that are nearly impossible to photograph well on land without some very sophisticated and expensive equipment. Like another other photography though, it does take a little preparation but the results can be very worth it.
Some things to consider when choosing a camera may be how often do you plan on using it, where (pool, lakes, ocean), how much time you wish to take in preparation of the camera, as well as ease of use underwater.
Probably the least expensive option is a disposable camera such as the Fujifilm model. These are typically single use cameras in a plastic housing. These may be ideal for snorkel trips or pool shoots. These a point and shoot cameras that produce an ok image, but certainly not the quality one may expect from some other models. Often very little preparation is needed with these cameras and are often ready to go right out the packaging. Control are often minimal if any at all and you are at the mercy of the cameras presets. Video is often not an option here.
There are other point and shoot cameras that produce a little better image such as the Fujifilm Finepix or of course the Go Pro water proof housing model cameras. These cameras are often moderately priced and are well suited for those who want a little more control of the final image. These cameras are often require little preparation (GoPro requires the housing), but controls are often small on the camera. Video is often possible which again leaves you with more options.
If you shoot with a DSLR, one choice for the camera that you already own is something like the DiCAP underwater housing which is essentially a rugged plastic bag enclosure with a specially fitted lens attachment on the outside of the bag. This can be useful for the occasional underwater photographer although can be someone difficult at times to control the camera settings. One must make sure the bag is carefully sealed according to directions or you could end up with a ruined camera. This units are best suited for pool use, and not recommended for dives more than about 10 feet.
Another choice for someone who may decide to shoot a little bit more often or go a little deeper with a camera the SeaLife camera can be a good choice. It is a dedicated sealed camera which requires very little preparation prior to use. It features a wide angle lens, color correction filters, large easy controls, video, as well as a myriad of other features. The results are typically very good, although some may consider the price a little high if you only use it once or twice per year.
Last but certainly not least are the professional camera housings such as Ikelite which may be used for deep dives for very serious underwater photographers. Once these housings enclose your camera properly (important), one can dive as much as 200 feet under the water!
It is probably best to practice in a pool first. Depending upon which camera you choose be certain to follow directions, if any on the camera prep. If you short cut, particularly with '0' rings, you may end up with a VERY expensive soggy paper weight! Shooting in a pool, you probably with have sufficient ambient light so strobes are not necessary. If you progress to deeper dives in open water, strobes are continuous lighting is almost a must.
Once in the pool, exhale and descend slowly. If you jump into the water it may take 10 or 15 seconds for the bubbles to clear before you can get a clear shot. You will need to be fairly close to your subject even in a pool to obtain clear images. Most dedicated underwater cameras have wide angle lenses, but if you are using your own DSLR, be sure that you have a wide angle lens mounted. Camera settings will of course vary depending on whether you have a sunny day or cloudy day, but an ISO of 400 and a shutter speed of 1/200 sec will often be enough to limit fuzzy pictures due to natural camera movement under the water. If you happen to be in open water, sand and other debris in the water is your enemy so you will need to be very quiet and once again close to your subject. A snorkel can help if you have one available. One other tip, when working in a pool it can be helpful to bring a backdrop (at least 10 ft wide). This can keep your images clean from unwanted pool reflections or tile designs.
Model photography can be very cool, but also difficult for both parties. Some of the same principles such as a slow descent, not disturbing sand or silt, and exhaling upon descent apply for the model. Free flowing clothing or fabrics are often popular to photograph. It can be VERY difficult for a model to open her eyes underwater especially if you are working in a chlorinated pool. Safety for you model should be the most important element here, so shallow dives (<10 feet) are best and even a safety diver may be helpful in you happen to be in open water (Open water -Do not recommend this for first time photographers or models). Specific poses are probably not going to happen, so be prepared to capture the best image that you can with a window after 15 seconds to about 45 seconds for most models. Again you will want to be relatively close to the model to capture clear images, but be careful about floating into each other!
Regardless of your camera, some post processing will be necessary. Color shifts are common with the skin appearing very blue or green. Some cameras such as the Sealife has built in filters, but many do not. There are MANY videos on Youtube on how to color correct such as this one from Lynda.com. You will often have to clean up which back scatter particles even in pools. One way to do this is to adjust the 'blacks' slider in Lightroom, or use the curves adjustment in Photoshop. Of course, in some cases the healing brush may also be helpful.
Be safe and I hope you found this helpful!
Bob Barford is a published photograher based in Sourthern Pennsylvania