I have a Canon T2i 18-55 setup. I was wondering what all equipment might I be needing to shoot underwater photos with this camera.
Is it possible to shoot underwater photos with these normal cameras with some additional gear?
Yes but you may not want to after knowing that most underwater housings cost far more than your camera.
Here is a housing designed for the T2i. You also need a matching Lens Port which has to be the right size for the lens you plan on using. Really, that is all you need. You can take it down to 100m.
If you plan to snorkel instead of scuba dive, you can probably find a compatible product from EWA Marine. There are basically thick extra-well-sealed ziplocks bags for your camera.
DISCLAIMER: I've never used EWA Marine products, only Sea & Sea. Aquatica apparently makes excellent ones too, one of my friends has been using one for years.
Depending on the importance of the underwater pictures and depth you want to take them at. You may want to consider simply buying a waterproof point-and-shoot which can go down to 10m. That is what I did after selling my expensive underwater photo gear realizing I had not done diving for 3 years straight.
It depends on a few factors. How deep do you wish to go with your camera? How much control do you want over the camera while underwater? Do you plan to use additional lenses besides your current lens?
A similar question has been asked before: Here
Without knowing the above, I can show you a few options that are available. Typically underwater housing units are quite expensive. B&H offers options with full camera controls in the range of $1399-$2100USD. Here is the most popular result - Ikelite6871.55 eTTL Housing f/ Canon EOS Rebel T2i. They also offer a more budget friendly option in the Aquapac SLR Camera Case, which runs only $139USD.
The huge difference between these two options is the depth at which they are rated to. The less expensive option will only allow you to head down about 15', while the more expensive ones upwards of 200'.
If you are just starting to look into underwater photography and do not want to invest a huge amount of money, you might be interested in a point and shoot style camera that is rated for underwater use(previous question), or even a GoPro video camera that takes still shots underwater.
There are already some good answers that explain the underwater housing you would need to make a camera like a DSLR capable of functioning under water. They can be expensive, and the more of your camera functionality you want to be able to control under water, the more expensive they tend to be.
One factor that is less often discussed when it comes to underwater photography is color balance. Water has the tendency to filter out "warmer" wavelengths of light, favoring the "cooler" wavelengths. This results in a significant blue shift. The problem is more complex than that, however, as it is less of a blue shift as it is a reduction in red, orange, and yellow light. This can affect the amount of natural color you can capture under water, resulting in very blue and more monochromatic images.
There are a couple things you can do to correct this. The first, and simplest, is to get some underwater flash or lighting. Something that is fairly powerful and emits a broad spectrum can help bring back some of the color that is lost because of filtering by the water itself. Which flash you get may depend on the kind of photography you wish to do. If your looking to photograph up-close macro shots, you might be able to make do with a cheaper underwater flash. If you want to photograph swimming fish or predators like shark at a distance, a more powerful (and often much costlier) flash would be necessary.
In addition to bringing down your own light, you can also correct the blue shift with your own filtration. Red, magenta and other warmer filters (sometimes called Magic filters) can be used to filter out a lot of the blue light, and re-balance color under water. It should be noted that by using any filtration, you are again reducing the amount of light present in the scene. You can compensate with higher ISO and faster lenses. Filtration is useful when you either can't afford underwater flash, or when underwater flash doesn't work for what you wish to photograph (such as large, wide-angle scenes...full reefs, wrecks, anything expansive or with greater depth than a flash can reach.) Filtration will usually still result in a fairly blue background, but it can fully correct the color balance of foreground.
It should be noted that digital cameras have the ability to automatically set white balance. In the case of the extreme blue shift you encounter under water, a camera's automatic white balance may not work particularly well, and and metering might also have some problems (digital sensors tend to be least sensitive to blue light.) You could try to get away with not using any physical filtration, shooting in RAW, and correct your white balance during post processing. Even with RAW, this is a pretty significant shift, and there are no real guarantees that you'll get the kind of rich color you probably want in your final pictures. If you use a digital camera, you should still consider using real underwater filters.
So, in addition to a proper underwater housing, you should plan to bring along some fast lenses and a bunch of underwater filters. The deeper you go, the more filtration you will need, so plan your depth ahead of time, and configure your lens with the right amount of filtration for the depth you wish to photograph at. Also keep in mind that the deeper you go, the more overall filtration your encountering, which will result in a greater and greater shift towards monochromatic tone.
Here are some links about underwater photography and lighting:
Basically it is possible but extremely cost prohibitive if you are trying to do anything but snorkel on the surface.
Any of those little velcro cases that go to really short depths are pretty scary to use with your proper gear, at depth the big problem with them is that the pressure will depress the buttons permanently whilst at that depth (an issue when at scuba depths as opposed to snorkeling on the surface) this can also get quite dangerous as the crushing forces will literally crush your camera (crack the lcd, break buttons) so stay within their allocated depths.
The big second issue is once you have it in a housing with an appropriate port, once you start breaking 5 meters you run into a problem where due to the physics of light through water, you lose the red spectrum very quickly. This is why a lot of photos underwater look very green, the way to combat this at depth and bring back the full colour spectrum is to use an underwater strobe (flash). These again become very expensive, the cheaper ones will have slower recycle times (one fire every couple of seconds) and have limited range both in distance and the angle of coverage. You can spend anywhere from a couple of hundred dollars to a couple of thousand and this is one area where the cost of the strobe directly relates to its performance and the quality of light you will get.
As said above in regards to a waterproof point and shoot, if you just want to share the experience and you will be leaving your camera in auto with the stock 18-55 kit lens, just get a point and shoot as the photo quality at that point will be mostly the same, if you just want to share the basic experience then that is what I would do. Any decent photo you see underwater is using extremely expensive and specialized gear for the specific shot.