water ice vs hoarfrost

water ice and hoarfrost both are nouns.

Word	Noun	Adjective	Verb	Adverb
water ice	Yes	No	No	No
hoarfrost	Yes	No	No	No

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

water ice: water frozen in the solid state
hoarfrost: ice crystals forming a white deposit (especially on objects outside)

Other hyponyms of water ice include black ice, frost, hoar, rime, hailstone, icicle, ground ice, segregated ice, Taber ice.

water ice (noun)	hoarfrost (noun)
an ice containing no milk but having a mushy consistency; usually made from fruit juice	ice crystals forming a white deposit (especially on objects outside)
water frozen in the solid state