Interesting article. I will try and pull the court cases mentioned.
https://sustainableatlantaga.com/2016/04/10/does-georgia-own-your-marshlands-yep-unless-it-or-the-king-of-england-gave-it-to-you/
In a quick scan I didn't see mention of the effect of changing extent of marshes. It would seem that the boundaries should have been set centuries ago and not change with current conditions. Is it a riparian boundary?
The main issue there is not the character of the land itself (that it is marshland), but that it is subject to the tides, making it tidelands. The US Supreme Court declared that tidelands are sovereign lands of the state in 1873 (Weber v. Harbor Commissioners, 18 Wall. 65), and in 1988 said that tidelands don't even need to be susceptible to actual navigation to be considered sovereign lands (Phillips Petroleum v. Mississippi, 484 US 469).
If these marshlands were above the head of tidal influence, there might not be any state claim (depends upon grant language and State, rather than Federal law).
TDGA, LLC v. CBIRA, LLC, Ga: Supreme Court 2016
http://www.gasupreme.us/wp-content/uploads/2016/02/s15a1638.pdf