sea vs South Sea

sea and South Sea both are nouns.

Word	Noun	Adjective	Verb	Adverb
sea	Yes	No	No	No
South Sea	Yes	No	No	No

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

sea: a division of an ocean or a large body of salt water partially enclosed by land
South Sea: any sea to the south of the equator (but especially the South Pacific)

sea (noun)	South Sea (noun)
a division of an ocean or a large body of salt water partially enclosed by land	any sea to the south of the equator (but especially the South Pacific)
turbulent water with swells of considerable size
anything apparently limitless in quantity or volume