the.com/high water mark

means The highest level ever reached by something — a river, a portfolio, an achievement — used as the benchmark that future performance must clear.

from Literal and nautical in root: the mark left on a shore or wall showing the highest point a flood or tide reached. River gauges and harbor walls were etched or painted with the actual line, so people could see in stone or wood exactly how high the water had risen. From there it waded into finance and general use — by the 1800s 'high-water mark' meant any peak never since equaled, and hedge funds later borrowed it literally: a fund manager only earns a performance fee on gains above the previous peak value, so investors aren't charged twice for the same climb.