I'm not one to normally give away secrets, but a good handful of photographers have asked me some technical questions about these shots, and it's honestly so simple that I figured I'd settle the record here and spill my technical beans about how I shoot underwater. I'm actually interested to see if anyone else tries this, and what kind of results you get. If you decide to try it, let me know!
Most people assumed I have an underwater housing for my camera. This is true, in a roundabout way. I simply went to Walmart and purchased an $8 dry box from the sporting goods section. It's just a clear blue waterproof plastic box that you would use on board a boat to store your keys, wallet, what have you. From there, it gets even simpler. I have a Canon A1000 point and shoot that takes video as well. I simply hit record on the camera, put it in the box, close the lid, and submerge it and swim. All of these shots are actually screen captures from a 5 minute video of Courtney flailing around underwater, drowning in a 10 foot length of black tulle and massive mounds of seaweed. :) I'm all about cheap fixes and improvising!
The only real drawbacks to this method are the fact that the size and resolution of the screen captures won't allow for good print quality, and of course there's the risk of getting your camera wet when using this setup, but I've found that keeping a small hand towel or washcloth in the box not only lets you cushion the camera against the front of the box for a clearer image, and you have something to wipe your wet hands on before you turn the camera off.
This can be done pretty much anywhere with clear water. I shot these in High Springs, FL, over a natural spring that constantly surges water. (Probably another reason Courtney was having a hard time swimming through, the force of the water pushes against you very hard.)