From Longman Dictionary of Contemporary Englishhigh water markˌhigh ˈwater mark noun [singular] 1 SCTTWthe mark that shows the highest level that the sea or a river reaches2 SUCCESSFULthe time when someone or something is most successfulhigh water mark of the high water mark of Herrera’s presidency
Examples from the Corpus
high water mark• I found what I took to be high water mark with my feet rather than my eyes.• It would provide the maximum area of water within the engineering constraints and would be broadly equivalent to mean high water mark.• But it was a misreading to suppose that the vote then marked the high water mark on the issue.• This was the high water mark of Elf culture when most of their greatest works of art were created. 