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]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "high water mark":
   Plimsoll line, Plimsoll mark, border line, bound, boundary,
   boundary condition, boundary line, bourn, break boundary,
   breakoff point, ceiling, circumscription, compass, confine, cutoff,
   cutoff point, deadline, delimitation, determinant, division line,
   end, extremity, finish, floor, frontier, hedge, interface, limen,
   limit, limitation, limiting factor, line, line of demarcation,
   load waterline, low-water mark, lower limit, march, mark, mete,
   start, starting line, starting point, target date, term,
   terminal date, terminus, threshold, tidemark, time allotment,
   upper limit, waterline, watermark