From WordNet (r) 2.0 (August 2003) [wn]:
highwater
n : the tide when the water is highest [syn: high tide, {high
water}] [ant: low tide]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]