Tidal doesn't necessarily mean the water is brackish.
Here in SC, our rivers are affected by tides well past the salt line.
Salt and freshwater have different densities.
Our intruding salt water (at high tide) pushes our freshwater back up the river, giving a full, freshwater fluctuation.
We don't have bogs, like other areas of the country.
The area this guy is speaking of is probably an old rice field that has grown up, giving him the impression of a swamp.
While many of our ancient rice fields have been restored for waterfowl hunting, we still have plenty abandoned. These fields have been growing cypress trees for over 200 years and are basically diked impoundments. His dikes have been broken for a while, allowing them to be affected by the high tides.
Eel grass, nor widgeon grass will survive here because both are submerged aquatics and need constant water to survive.
teal and tailing reds make life worth living