hot-water bag vs hot-water bottle

hot-water bag and hot-water bottle both are nouns.

Word	Noun	Adjective	Verb	Adverb
hot-water bag	Yes	No	No	No
hot-water bottle	Yes	No	No	No

As nouns, hot-water bag and hot-water bottle are synonyms defined as:

hot-water bag and hot-water bottle: a stoppered receptacle (usually made of rubber) that is to be filled with hot water and used for warming a bed or parts of the body

hot-water bag (noun)	hot-water bottle (noun)
a stoppered receptacle (usually made of rubber) that is to be filled with hot water and used for warming a bed or parts of the body	a stoppered receptacle (usually made of rubber) that is to be filled with hot water and used for warming a bed or parts of the body