hydromancy

from WordNet (r) 3.0 (2006)

hydromancy
    n 1: divination by water (as by patterns seen in the ebb and
         flow of the tides)

from The Collaborative International Dictionary of English v.0.48

Hydromancy \Hy"dro*man`cy\, n. [Hydro-, 1 + -mancy: cf. F.
   hydromancie.]
   Divination by means of water, -- practiced by the ancients.
   [1913 Webster]