A lot of good replies to NCfowls question. And Too Tall correctly points out that the impoundments in question are actually Submerged Aquatic Vegetation (SAV) units. Moist-soil units are managed much differently.
Water salinity is the driving factor as to how each unit is managed in the coastal areas. Corn simply will not do well with residual salt in the soil. Some moist-soil plants/units can withstand some salt (around 5 ppt) but overall will not like having that much if looking for a diversity of plants the following year. By contrast, your SAV such as milfoil, wigeon grass, musk grass, etc. does fine in the area brackish waters and therefore are the best match for effective and cost efficient management by the WRC. They don't have to be drawn down every year like a ms unit would, but do periodically in order to stimulate new growth and to firm up the bottom. Remember, all those units were basically built in a marsh so you should expect the bottoms to be soft, especially where root establishment is thin.
As to the corn impoundments in that area, I think Splash'n Divers pretty much nailed it when he said that more birds will be taken on the sav units. Lots of reasons why but the vast majority of the birds that use/are taken in the area such as wigeon, pintail, gadwall feed almost exclusively on sav plants. Teal, which actually might be the #1 bird taken, probably prefer seed/grain above anything else but can dominate the sav units at times. A corn unit is also very expensive to plant, flood and maintain. Wells are the preferred way to flood corn vs pumping from a brackish canal. The cost to dig a well will blow your mind, let alone the cost of the pump and the subsequent electric bill. Ouch! Oh, and did I mention the approximate $300 per acre cost just to get corn in the ground? It all gets very expensive quickly ...
While all of this just my take on it, I hope you'll find some this info helpful.