water mint vs water-mint

water mint and water-mint both are nouns.

Word	Noun	Adjective	Verb	Adverb
water mint	Yes	No	No	No
water-mint	Yes	No	No	No

As nouns, water mint and water-mint are synonyms defined as:

water mint and water-mint: a European mint that thrives in wet places; has a perfume like that of the bergamot orange; naturalized in eastern North America

Other synonyms of water mint include Mentha aquatica.

water mint (noun)	water-mint (noun)
a European mint that thrives in wet places; has a perfume like that of the bergamot orange; naturalized in eastern North America	a European mint that thrives in wet places; has a perfume like that of the bergamot orange; naturalized in eastern North America