This information is aimed at those that have not researched this subject before, and will not likely help those that have done any previous underwater photography.  Underwater photography is a big compromise when compared to regular photography, due in part to the equipment, but mainly because you are very limited in what is possible. Many of the problems are at depth, so for snorkelers they are not a big issue, and you can get decent results at a minimul cost. You can buy several brands of disposable underwater cameras for not that much more than regular disposables. Some dive shops also sell inexpensive film cameras that you can reload with new film, but they are only rated waterproof in shallow water, and not suitable for diving.

The photos above were taken while snorkeling in shallow, but silty water, (due to constant wave activity). and in order to get profiles I dove down to 5 or 6 feet before taking the shots. If taken without diving down they would not be as interesting.

There are differences between some disposables regarding the maximum depth they are rated to, and this should be considered when shopping for them, using the intended use as a guidline. If you are just going snorkeling, be sure to wait until you arrive at your destination before buying your disposible camera and don't stock up.  Buy only 1 at a time as you need them.  If you buy them before you leave home, be sure to pack them in your carry-on for minimum x-ray exposure to the film.  (Best to wait 'til you get there so there is one less time the film is exposed to security x-ray machines -- they will cost more than back home, but the quality of your photos will be much better if the film is kept to a minimum of times exposed to the x-ray machines.)  NEVER pack your film or cameras with film in your checked luggage.  The x-rays that scan checked bags are much heavier than the x-rays for carry-ons and your film can be ruined if put in your checked bags.  Many disposibles are rated good to a depth of 15 to 20 feet, while others can be used to 40 to 50 feet according to their specifications. The 15-20 ft. kind are great for snorkeling pics.  Some divers use the  deeper rated versions where they have taken them much deeper than the rating, with the following result (observed but not personally done); they implode as the depth increases preventing you from advancing the film, but if it was advanced prior to passing the rated depth, they generally can take one shot, and when you ascend back to the rated depth, or less, they seem to work properly again. This is not a guarantee it will work for you. With all the advances in digital cameras and inexpensive underwater housings for several of them, it is becomming very common to see them used by a lot of people.  If you are considering taking your digital camera snorkeling or diving you must be very careful when you close up the housing, as even something as small as a grain of sand or a single strand of hair that gets caught in the seal can cause leakage. Cameras and water don't belong together, and if it is salt water the camera becomes a conversation piece instead of a camera.

This pair of photos show the bluish tint that underwater photos get, caused by many variables, but mostly by the fact that water filters out the normal light spectrum. You should also notice that the depth of field diminishes too, so telephoto lens settings are pretty much useless underwater. Underwater photography is usually done with wide angle lenses, or that setting on a zoom lens. Please note that these shots are "as taken" and I have not used any software to alter the colors. Before uploading them I did try enhancing them and was able to drastically, but not fully, remove the bluish tint just by raising the color temperature.  

For serious underwater photography you can spend a very large amount of money, and will need to practice constantly to get your skills perfected. Reasons that it is rather hard to get good results (compared to regular photography) is that you can't hold the camera perfectly still in relation to your subject since you are floating, or even if able to lay on the bottom, you are generally moved by wave surge. Add to that the fact that many of the things you are trying to photograph swim erratically, and are afraid of you, or are also caught by wave surge. Other factors that reduce your image quality are particles suspended in the water (silt) and the fact that as you go deeper different colors of light are filtered out by the water, until almost all color is gone. Underwater photos generally have a bluish cast, and although digital images can be altered to help correct some of it, they will still be bluish. Knowing that the colors have fadded away at depth prompts you to turn on your built in flash, but if the water is silty then the photo is filled with backscatter, which looks like a snowstorm. Luckily most digital cameras can take decent photos in low light without a flash, and using software you can work wonders to the photos compared to what was possible with film. For serious underwater flash photography divers buy strobes (flash units) on adjustable arms that allow placing the flash (or flashes) at an angle that prevents backscatter, but both adds lots of cost, and makes the equipment much more bulky.

This pair of photos shows the backscatter that your camera will capture if the built in flash is used in silty conditions. If you look carefully at the lower shot you will see a camera with a remotely mounted flash on the left edge of the photo which when used reduces the problem drastically.

Proper underwater film cameras do exist, and now that so many divers are going digital there are many sources for used ones at reasonable prices. If they interest you then be certain that they work, and haven't been flooded. You will need the owners manual, and especially the instructions for closing the housing and a source for new O rings to seal the housing in future years. The big draw to digital over film is the ease of enhancing the images on a computer. Unless you do your own darkroom work you will be totally at the mercy of the photo lab, which is okay when the photos were shot in shallow water with good light, but at depth they will be very blue.

Here you see what can be done when it all goes according to plan. That strange thing is a Frogfish, and they are not common, so getting a good photo was important. I was able to get close enough to use the built in flash, and due to depth it was necessary or the photo would have had almost no color.  

Another of the hurdles that you need to overcome when shooting underwater that adds to the difficulty in capturing that perfect shot. On land you can hold the viewfinder to your eye, or use a built in LCD screen to clearly see how you have framed your target. Underwater you have a mask on that prevents that, and more importantly you need to try and get the camera closer to the subject if it is a fish, turtle, or eel etc, without scaring it, so you must stretch out your arms with the camera and guess where to aim it. This is not the best way for perfect framing, so it is a good idea to use a wide angle setting if you have the choice, and try several shots hoping for at least one keeper. 

In regard to one difference between snorkeling or diving,  if possible you should secure your camera to you, either by a wrist strap or a lanyard cable that you can clip to your bathing suit somehow.  Another very important consideration is camera buoyancy, which for snorkeling shouldn't be a problem, as whatever type you have chosen likely will float. When diving you want the camera to be neutral, and not capable of floating on the surface. It is rather important that you either buy the correct weight set for your camera and housing, or make your own. If you don't do this, you will have problems controlling it, and your own buoyancy underwater, and more importantly if it gets away from you it will head for the surface very quickly, much faster than you can do safely. If you don't have access to a pool, a big bucket of water should be used, and the ideal situation has the camera and housing just barely float, or at least not sink when released below the surface.  

I think the set of photos above can demonstrate why digital has quickly  become the chosen method of underwater photographers. I was able to alter the first (original as shot) using a very easy to use free program called Picasa 2 in less than a minute per new version. Add to that the fact that the original still stays undamaged, and available for even more variations, and at no cost. In the darkroom this would have taken several trial & error exposures, and could have taken up several hours time, plus used up several sheets of photo paper & the needed chemicals.       

This link is to an on line manual from Canon Cameras that is very useful at explaining basic procedures for digitals, but since it compares several things to film you can interpret them backwards to use with a disposable or regular camera. You can read it on line, or print it off as desired, and includes a section of how to maintain your housing.     http://www.canon.co.jp/Imaging/uwphot   This link is to a more in depth article about equipment on the B & H Photography web site  http://www.bhphotovideo.com/bnh/contr...