Of course the Gulf of Mexico is not particularly pristine.

Farm Runoff- Swaths of the Gulf near the Louisiana Delta are Oxygen-starved (hypoxic) because of the farm runoff of fertilizers, feeding algae blooms which consume the oxygen in the water. Fish leave and bottom fauna such as crabs die.

Erosion- Coastal wetlands are wasting away because they no longer receive river sediments; the Mississippi is constricted by levees starving the marshes.
Coastal lands are sinking; oil and water removal have accelerated the process.

Trash- Along Padre Island alone, 150 tons of trash are collected annually by volunteers.

Oil Spills- from ships, damaged pipelines, well operations. Oil companies reported that drilling platforms annually dump 50,000 barrels of oil.

Population Increase- Population of coastal counties increased 45 percent from 1980-2003.

NY Times, Environment page, May 9, 2010.