The Collaborative International Dictionary of English v.0.48:

Tidemark \Tide"mark\, n.
   1. a mark on a shore line indicating the normal level of the
      water at high tide, caused by the action of the flowing
      water; sometimes also used for the mark left at the point
      of the normal low tide; -- called also high water mark.
      [PJC]

   2. [fig.] the maximum level which some activity has reached
      and from which it has declined; -- called also high water
      mark.
      [PJC]

WordNet (r) 3.0 (2006):

tidemark
    n 1: indicator consisting of a line at the highwater or low-
         water limits of the tides