sea vs head sea

sea and head sea both are nouns.

Word	Noun	Adjective	Verb	Adverb
sea	Yes	No	No	No
head sea	Yes	No	No	No

As nouns, head sea is a hyponym of sea; that is, head sea is a word with a more specific, narrower meaning than sea:

sea: turbulent water with swells of considerable size
head sea: a sea in which the waves are running directly against the course of the ship

sea (noun)	head sea (noun)
a division of an ocean or a large body of salt water partially enclosed by land	a sea in which the waves are running directly against the course of the ship
turbulent water with swells of considerable size
anything apparently limitless in quantity or volume