high-water mark

/HH AY1 W AO0 T ER0 M AA0 R K/
noun
  1. 1

    A mark, such as a line of seaweed, showing the highest level reached by a body of water.

  2. 2

    (by extension) The peak or apex of something.

Source: https://en.wiktionary.org/wiki/high-water%20mark