filter vs water filter

filter and water filter both are nouns.

Word	Noun	Adjective	Verb	Adverb
filter	Yes	No	Yes	No
water filter	Yes	No	No	No

As nouns, water filter is a hyponym of filter; that is, water filter is a word with a more specific, narrower meaning than filter:

filter (noun)	water filter (noun)
device that removes something from whatever passes through it	a filter to remove impurities from the water supply
an electrical device that alters the frequency spectrum of signals passing through it

filter (verb)	water filter (verb)
remove by passing through a filter
pass through
run or flow slowly, as in drops or in an unsteady stream