Sedges and Reeds are superficially similar, but actually quite different. But unless you are a biologist - who cares? ( Reeds are grasses, sedges are sedges. )
Sedges
Sedges tolerate a wide range of salinity, from freshwater to seawater, although they seem to grow best in brackish water. So, like above, if you are exploring a bay or inlet, that stuff all around you is most likely sedge.
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