During the American and Civil Wars, the salt supplies from overseas and the Caribbean were cut off by naval blockades. The Chesapeake is one of the largest estuaries in the world. Marine fish, species who can stand salt water, and freshwater species both take advantage of the estuary. In addition, numerous plant and animal species have specialized over the centuries to thrive in the brackish water. The biodiversity as well as the productivity of the Bay is high.
Salt is the key.
If the concentration of salt is too high, it can stop biological action - as in the barrels of George Washington's salted herring. Pickle a cucumber, and it will last far longer than a fresh one. Look at the pictures of Chincoteague and other Tidewater areas at sea level, and you'll see marches filled with grasses. Photosynthesis in an acre of Chesapeake Bay salt marsh can easily surpass the productivity of any Virginia farm in the Shenandoah Valley. Humans prefer to eat corn on the cob rather than marsh grasses, of course, so few of us normally think of salt water marshes as more valuable than corn fields.