Where you can actually use the camera and that will go below snorkeling depths?
Yeah, I know it's not a real camera, and you wouldn't use it for any professional work, but you can get some decent quality stuff out of the iPhone 4. It has it's uses, and it fits in a pocket.
The iDive really seems to be the only housing for it, but only allows iPod controls.
Only those bag style things, that can only got to about 20' let you use the camera.