In a way, it's both, but fiddler crabs, for instance, live in brackish water.
From Wikapedia:
Since the bay is an estuary, it has fresh water and brackish water. Brackish water has 3 salinity zones, oligohaline, mesohaline, and polyhaline. The salinity is measured on parts per thousands (ppt). The fresh water zone runs from the mouth of the Susquehanna River to north Baltimore. The oligohaline zone has very little salt. Salinity varies from 0.5ppt-10ppt and freshwater species can survive there. The north end of the oligohaline zone is north Baltimore and the south end is the Chesapeake Bay Bridge. The mesohaline zone has a medium amount of salt and runs from the Bay Bridge to the mouth of the Rapahannock River. the salinity ranges from 10.7ppt-18ppt. The polyhaline zone is the saltiest zone and some of the water can be as salty as sea water. It runs from the mouth of the Rappahannock River to the mouth of the bay. The salinity ranges from 18.7ppt-36ppt. 36ppt is as salty as the ocean.
Nancy