A Salt Marsh is typically brackish water and the salinity fluxuates quite frequently. These types of tanks are quite common in the brackish aquarist community. The best setup would be a paludarium only 1/2 filled with water. Sea Grasses could be grown above the water line and an active tidal system could be built to simulate an actual tide schedule. I've been doing something like this with my 200G Archerfish tank which is still in the process of being built.
The neat thing about doing a Southeast Coast Salt Marsh tank is the ability to use subtropical fish. You'd probably not even use a heater and maybe get away without the use of a chiller depending on where you live. Collecting species would be the most difficult part. It's probably not legal to collect fish for that purpose without some sort of permit.
However, I can recommend Sach's Aquaculture Systems @
http://www.aquaculturestore.com for supplying your livestock. This is an excellent establishment with an even better staff. I have gotten two Hogchoker Soles from them, shipped from Florida to California, both arrived in excellent shape. They specialize in US Natives and carry a few brackish subtropical species.
If it were me, and one day it might be, I'd do a large seagrass tank with a mud substrate and a HUGE school of silversides. Throw in a few crabs and shrimp to inhabit the seagrass bed, maybe a hogchoker, and leave it at that.
Whatever you do just ensure that all of the livestock share the same salinty requirements. Brackish isn't a specific salinity, it's technically everything inbetween 1.000-1.022SG, even though 1.017SG can be considered FO Marine.
