The Collaborative International Dictionary of English v.0.48:

Cotidal \Co*tid"al\ (k?-t?d"al), a.
   Marking an equality in the tides; having high tide at the
   same time.
   [1913 Webster]

   Cotidal lines (Phys. Geog.), lines on a map passing through
      places that have high tide at the same time.
      [1913 Webster] Cotillon