OK, I don't find this a particularly credible Doomsday Scenario, but just in case, a hearty f-u to all the environmental doubters out there.
If the methane bubble—a bubble that could be as big as 20 miles wide—erupts with titanic force from the seabed into the Gulf, every ship, drilling rig and structure within the region of the bubble will immediately sink. All the workers, engineers, Coast Guard personnel and marine biologists participating in the salvage operation will die instantly.
Next, the ocean bottom will collapse, instantaneously displacing up to a trillion cubic feet of water or more and creating a towering supersonic tsunami annihilating everything along the coast and well inland. Like a thermonuclear blast, a high pressure atmospheric wave could precede the tidal wave flattening everything in its path before the water arrives.
When the roaring tsunami does arrive it will scrub away all that is left.
And oh yeah, the methane could trigger all other sorts catastrophes, including extinction of most life on earth.