head sea vs sea

head sea and sea both are nouns.

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

As nouns, sea is a hypernym of head sea; that is, sea is a word with a broader meaning than head sea:

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

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