I dont think some people really understand just how much sea food comes out of the Gulf, and it is a relatively contained body of water even as large as it is.
Since this one is a deep water uncontainable event we have no way of knowing just how much it is out there nor how much oxygen depletion will occur. We do know that there is enough oil/benzene etc out there to kill whatever it comes into contact with right now.
If all else fails, the relief wells will contain it.