The Collaborative International Dictionary of English v.0.48:

highwater \highwater\, high water \high water\n.
   1. the tide when the water is highest; high tide. [WordNet
      sense 1]

   Syn: high tide, high water.
        [WordNet 1.5]

   2. Water at its highest level, as in a river during a flood.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

high-water \high-water\ a.
   Pertaining to water at its highest achieved level; of or
   pertaining to high water; as, the high-water marks on the
   walls after a flood.
   [PJC]

WordNet (r) 3.0 (2006):

highwater
    n 1: the tide when the water is highest [syn: high tide, high
         water, highwater] [ant: low tide, low water]