The deathbox a term given to a shelf, or sitting area of a pool. The correct term for it is "swimout." (Thanks, C-Dawg) It is typically a few feet below the surface of the water, and is usually set back from the basic shape of the pool.
This term is used mainly by skateboarders, when skating pools. They call it this, because such a platform, usually about 3-5 feet across, is just the right size to try and get air over. Not impossible, but still quite challenging. And, of course, if you bail, it hurts a lot.
There also used to be a small skateboard company called Deathbox. They almost went under, until they were bought out by Blitz Distribution, and had their named changed to The Firm. Now, Bob Burnquist rides for them, among others, and they are doing quite well.