The Liquid Image is a relatively inexpensive snorkeling mask with a built-in digital camera, available in 3.1- and 5-megapixel versions starting at $100. Tiny crosshairs on the mask give you some idea of how well framed your picture may be, while embedded LEDs let you know the camera's in picture-taking mode. A MicroSD slot offers memory expansion.
Besides the obvious issues of getting the focus correct, I think it's pretty neato. It's certainly one less thing to try to carry when you're in the water, although it's a pity it's only rated to 15 feet. (The camera, I presume, not the mask.)
It looks like the mask is being made by a toy company, too, which usually means the price drop will be fairly swift. I wouldn't mind trying one of these out.