I have a few thoughts on this. I think the gulf and Lake P can handle all the polution from this mess without a real environmental disaster.
On the other hand, keeping a large non-moving area of water that is contaminated with feces, dead and decaying bodies, and other crud will be totally devastating. Mosquito born illnesses will certainly be on the rise.
Not to mention the stench that must be coming from that stagnant water.
The Gulf is already extremely polluted. When you fly into the United States from the south, the water is a beautiful aquamarine... until you get close to the US. It turns to an ugly brown with the shimmering green and blue on top. It looks much like the toxic soup bowl of NO right now. This would make environmentalist's hair stand on end, and I'm not expert, but I can't see this bit of water harming the Gulf worse than it already is. Just pump it and be done with it.
http://wjbo.com/audio/katrina/AlSharpton.mp3
WWL interview Al Sharpton
http://wjbo.com/audio/katrina/GarlandRobinette-RayNagin.mp3
WWL interview Ray Nagin