The Collaborative International Dictionary of English v.0.48:

Hydrosphere \Hy"dro*sphere\, n. [Pref. hydro-, 1 + sphere.]
   1. (Meteor.) The aqueous vapor of the entire atmosphere.
      [Webster 1913 Suppl.]

   2. (Phys. Geog.) The aqueous envelope of the earth, including
      the ocean, all lakes, streams, and underground waters, and
      the aqueous vapor in the atmosphere.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

hydrosphere
    n 1: the watery layer of the earth's surface; includes water
         vapor

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "hydrosphere":
   agua, aqua, big drink, blue water, drink, drinking water, eau,
   ground water, hard water, head, heavy water, high sea, high seas,
   hydrol, hydrometeor, limewater, main, main sea, mineral water,
   ocean, ocean depths, ocean main, ocean sea, rain, rainwater,
   salt sea, salt water, sea, sea water, soft water, spring water,
   steam, thalassa, the bounding main, the brine, the briny,
   the briny deep, the deep, the deep sea, the seven seas,
   the vasty deep, tide, water, water vapor, well water,
   wetting agent, wetting-out agent