Source : Webster's Revised Unabridged Dictionary (1913)

Hydromancy \Hy"dro*man`cy\, n. [Hydro-, 1 + -mancy: cf. F.
   hydromancie.]
   Divination by means of water, -- practiced by the ancients.

Source : WordNet®

hydromancy
     n : divination by water (as by patterns seen in the ebb and flow
         of the tides)