Clocking in at over 4,000 acres, the marshlands make up a majority of Sea Rim State Park. The unit is made up of both brackish and intermediate marshes. Brackish marshes are moderately saline (salty), while intermediate are nearly fresh. Because of this unique combination, these marshes have a wider variety of plants than do purely salt marshes. Plants like Gulf cordgrass and phragmite are common and grow in the marsh here.
The marsh unit has lagoons and salt meadows, tidal drainages and two lakes, Fence and Salt Lakes. The marshes provide important breeding and foraging habitat for many species of shell and fish.
The Marsh unit borders J.D. Murphree Wildlife Management Area.