A river inlet does not really have any synonyms. Though a bay or recess or a narrow strip of water running into the land or between islands, have close similarities.
I know a couple examples, all of which are in Long Island, NY: Shinnecock Inlet Rockaway Inlet East Rockaway Inlet Fire Island Inlet Moriches Inlet
The plural is bays.
They are synonyms.
Synonyms
synonyms for prominence:importancefamestatureprestige
Estuary - meaning the part of a river which opens and meets the sea, has many synonyms. These include but are not limited to: arm, creek, firth, fjord,inlet and waterway.
bay, sound, inlet, anchorage
inlet, mouth, creek, firth, fjord
Indian River Inlet Bridge was created in 2012.
The marshy inlet of a river or lake is called a bayou. The name for an artificial waterway is a canal.
Inlet or outlet
Bayou
Sebastian Inlet is located in Sebastian Inlet State Park. Sebastian Inlet State Park is located in Brevard County Florida and Indian River County, Florida.
Ravija
estuaries
An estuary is an inlet of water where a river empties into an ocean or sea. It is formed from the freshwaters of rivers and stream, protected from storms, ocean waves and winds.
river