high water vs neap

high water and neap both are nouns.

Word	Noun	Adjective	Verb	Adverb
high water	Yes	No	No	No
neap	Yes	No	No	No

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

high water: the tide when the water is highest
neap: a less than average tide occurring at the first and third quarters of the moon

Other hyponyms of high water include direct tide, neap tide, springtide.

high water (noun)	neap (noun)
the tide when the water is highest	a less than average tide occurring at the first and third quarters of the moon