Well it depends on how much underwater photography you want to do, and also how deep you plan to go. There are cases for your good DSLR that allow you to take them underwater, however they are very expensive, often same or more as a new camera.
I know a lot of people that will take a simple Point and Shoot camera, like Canon powershot 570 or 720 and then put it in a good clear plastic baggy and then secure the baggy so there is as little space between the baggy and the lens so the images come out best. Note, that you will need to set it to like ISO 800 before you go in the water, and that it all will have a blue cast to it, but that can be corrected for in Photoshop.
If you watch Ebay you will see cases for the Canon 20D and 30D sometimes and even a few Rebel xt cases and last week I saw one for a 5D but the case was well over $1200 used.
I would suggest seeing what you like and try the point and shoot first. should work to about 20-30 ft my buddy says w/o any problems.
Hope this helps. Good Luck and remember, dont swim for 2 hrs after eating...!!!.
Many manufacturers make cameras that work underwater, but at most camera shops, you should be able to find a waterproof housing that may work with your existing camera. You do have to be careful that you do not go under deeper than the rated depth of the camera or housing. Some of those housings are not much more than shields to prevent damage from being splashed, and wont work beyond a few feet under.