You will need: 1. a rift system and 2. some time.
Consider the real world Jordan rift valley:
(image credit: Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC)
The dark blob on the left is the Mediterranean sea. The smaller dark blob surrounded by green at the top right is the Sea of Galilee, a freshwater lake. The larger dark blob at the center right is the Dead Sea, a famously hypersaline lake.
There is a theory that the Dead Sea was filled by flooding from the Mediterranean. It lies in a pretty unfriendly desert these days, but one could imagine a period in the life of such a lake where it was merely the regular kind of saline, and as its parent sea retreated but before it because too salty a little isolated salt-water ecosystem might arise. With the right conditions, it might transition to being hypersaline sufficiently slowly than halophytic plants and wildlife could arise there.
But that's not the only way... quoth the wikipedia page,
The Sea of Galilee is at risk of becoming irreversibly salinized by the salt water springs under the lake, which are held in check by the weight of the freshwater on top of them.
Salt water springs! Such things might let you have an inland saline body of water without it needing to also be an endorheic basin.
I also see references to places like Zuni Salt Lake, a lake that formed in a volcanic basin being not only quite salty but also having their own wildlife. Clearly the things you're requiring are not totally implausible, and there are multiple ways for them to arise.
And indeed, I've found something that is explicitly an inland saltwater marsh: Quivira National Wildlife Refuge. It is in Kansas, which is not particularly close to the sea. According to the Kansas Geological Survey, the salts come from underground salt deposits (eg. salty springs) and whilst the water is very brackish it isn't as saline as seas generally are. There's plenty of scope for handwaving, anyway. Unlike other salty lakes (especially in endorheic basins) it has vegetation, not just extremeophilic cyanobacteria, which probably makes it a bit nicer to visit.