brackish vs briny

brackish is not a noun while briny is a noun.

Word	Noun	Adjective	Verb	Adverb
brackish	No	Yes	No	No
briny	Yes	Yes	No	No

As adjectives, brackish and briny are synonyms defined as:

brackish and briny: slightly salty (especially from containing a mixture of seawater and fresh water)

brackish (noun)	briny (noun)
	any very large body of (salt) water

brackish (adjective)	briny (adjective)
slightly salty (especially from containing a mixture of seawater and fresh water)	slightly salty (especially from containing a mixture of seawater and fresh water)
distasteful and unpleasant; spoiled by mixture