lifesaver vs flotation device

lifesaver and flotation device both are nouns.

Word	Noun	Adjective	Verb	Adverb
lifesaver	Yes	No	No	No
flotation device	Yes	No	No	No

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

lifesaver: a life preserver in the form of a ring of buoyant material
flotation device: rescue equipment consisting of a buoyant belt or jacket to keep a person from drowning

Other hypernyms of lifesaver include life preserver, preserver.

lifesaver (noun)	flotation device (noun)
a life preserver in the form of a ring of buoyant material	rescue equipment consisting of a buoyant belt or jacket to keep a person from drowning
an attendant employed at a beach or pool to protect swimmers from accidents