One of the issues is that to drill all the wells back in the marshes, the oil companies would dredge channels back to the location so that a barge rig could be used to drill the well. It is too swampy for a conventional land- based rig. These channels allow salt water to encroach into the swamp and it kills the marsh. Without the vegetation, the silt washes away.
The Mississippi River Gulf Outlet (MrGo) did the same thing. It let in seawater which ruined the wetlands surrounding it.