Its a bad idea, because you are taking the saltwater marsh away from the ecosystem... And turning it into a cement covered ground, for more houses... Then the animals (or other things) that live there, will die... and have to find another place to live...
~therefore... The bad thing is, that they shouldnt fill the marsh with dirt, and cement... Because bad things could happen to the creatures that already live there.
The Biosphere is the answer that ur looking for
