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]WordNet (r) 3.0 (2006):
hydromancy n 1: divination by water (as by patterns seen in the ebb and flow of the tides)