When an area is referred to as 'The Dead Zone,' it's a fair indication that you probably never want to pay it a visit. In this case, the dead zone is out at sea, and the dead are all of the fish and other aquatic life in the affected part of the Gulf of Mexico.
The Dead Zone is the most notorious hypoxic area in all of the United States. It's the result of years of phosphorus and nitrogen being dumped out at sea by the Mississippi River. The waste didn't end up here entirely on purpose; the Mississippi River has the unfortunate honor of being the primary drainage area for almost half of America. Waste dumped into lakes and streams from many different locations finds its way into the river, then out to sea, and then out into the dead zone, which covers 8000 square miles. The chemicals sap the oxygen level in the water, meaning the fish can't breathe. They end up washed up on beaches like this.