saltwater vs H2O

saltwater and H2O both are nouns.

Word	Noun	Adjective	Verb	Adverb
saltwater	Yes	No	No	No
H2O	Yes	No	No	No

As nouns, H2O is a hypernym of saltwater; that is, H2O is a word with a broader meaning than saltwater:

saltwater: water containing salts
H2O: binary compound that occurs at room temperature as a clear colorless odorless tasteless liquid; freezes into ice below 0 degrees centigrade and boils above 100 degrees centigrade; widely used as a solvent

Other hypernyms of saltwater include water.

saltwater (noun)	H2O (noun)
water containing salts	binary compound that occurs at room temperature as a clear colorless odorless tasteless liquid; freezes into ice below 0 degrees centigrade and boils above 100 degrees centigrade; widely used as a solvent