Underwater Photography.

I have never really understood how this works.  I guess i assumed that you had to have one of those big Jacques Cousteau cameras on wheels (they look like little hotdog carts).  Turns out, that's not true.  Of course.

 

Here is a really complete guide to basically all things underwater photography:

Underwater Photography Guide

And then there's this rundown, which is better suited to my attention span, courtesy of Photo.net:
Underwater Photography: a primer

And here are some underwater housings for different sorts of cameras:
Underwater Camera Housings

And here is just one of several AWESOME underwater photography groups on Flickr:
Underwater Portraiture Group

If you'd prefer not to have to read all of that it basically goes like this:

* buy a waterproof camera and then use it.

OR

* buy a waterproof housing for whatever kind of camera you like to use.  it seems like the more expensive the camera the more expensive the housing will be but pretty much all of them come with a guarantee to the effect that they will replace your camera if the housing doesn't protect it.

* go underwater

* take pictures, remembering that light is refracted differently and stuff like depth of field is going to be pretty crazy.