The Collaborative International Dictionary of English v.0.48:

Hydropathy \Hy*drop"a*thy\, n. [Hydro-, 1 + Gr. ?, ?, to
   suffer.]
   The water cure; a mode of treating diseases by the copious
   and frequent use of pure water, both internally and
   externally.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hydropathy
    n 1: the internal and external use of water in the treatment of
         disease [syn: hydropathy, hydrotherapy]