high water

plural high waters

1. uncountableThe location of high tide on a coastal area.

   So as not to impede navigation the two principal spans of the bridge were designed to provide a clear headway of 157 ft. above high water for a distance of 500 ft.; [...].

   1941 January, the late John Phillimore, “The Forth Bridge 1890-1940”, in Railway Magazine, page 5

2. countableuncountableThe state of the tide when the water is at its highest.

   One of the requirements of the design was the provision of a clear headway of 50 ft. at high water for shipping.

   1952 September, “Chepstow River Bridge Centenary”, in Railway Magazine, page 623

3. countableThe highest stage of a river.