high water vs neap tide

high water and neap tide both are nouns.

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

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

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

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

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