breeches buoy vs lifesaver

breeches buoy and lifesaver both are nouns.

Word	Noun	Adjective	Verb	Adverb
breeches buoy	Yes	No	No	No
lifesaver	Yes	No	No	No

As nouns, lifesaver is a hypernym of breeches buoy; that is, lifesaver is a word with a broader meaning than breeches buoy:

breeches buoy: a life buoy in the form of a ring with short breeches for support; used to transfer people from a ship
lifesaver: a life preserver in the form of a ring of buoyant material

Other hypernyms of breeches buoy include life belt, life buoy, life ring.

breeches buoy (noun)	lifesaver (noun)
a life buoy in the form of a ring with short breeches for support; used to transfer people from a ship	a life preserver in the form of a ring of buoyant material
	an attendant employed at a beach or pool to protect swimmers from accidents