The Collaborative International Dictionary of English v.0.48:

Hydroplane \Hy"dro*plane\, n. [Pref. hydro-, 1 + plane.]
   1. A plane, or any of a number of planes, projecting from the
      hull of a submarine boat, which by being elevated or
      depressed cause the boat, when going ahead, to sink or
      rise, after the manner of an a["e]roplane.
      [Webster 1913 Suppl.]

   2. A projecting plane or fin on a hydroplane[3] to lift the
      moving boat on top of the water; also, a gliding boat.
      [Webster 1913 Suppl.]

   3. a powered surface boat fitted with planes (hydroplanes[2])
      projecting below the hull, designed to lift the boat, when
      moving rapidly, so that the hull itself is lifted out of
      the water and the boat is supported by the hydrodynamic
      forces of the water on the hydroplanes[2], thus permitting
      higher speeds; formerly called a gliding boat.
      [Webster 1913 Suppl. +PJC]

The Collaborative International Dictionary of English v.0.48:

Hydroplane \Hy"dro*plane\, v. i.
   1. Of a boat, to move through water while supported by
      hydroplanes[3] (see hydroplane[3], above).
      [Webster 1913 Suppl.]

   2. to move through a body of water supported by the
      hydrodynamic forces on a surface, similar in principle to
      a hydroplane[3]; -- said, e. .g, of automobiles skidding
      on a shallow patch of water on a road when moving at high
      speed, thus causing the tires to lose contact with the
      road surface.
      [PJC]

WordNet (r) 3.0 (2006):

hydroplane
    n 1: an airplane that can land on or take off from water; "the
         designer of marine aircraft demonstrated his newest
         hydroplane" [syn: seaplane, hydroplane]
    2: a speedboat that is equipped with winglike structures that
       lift it so that it skims the water at high speeds; "the
       museum houses a replica of the jet hydroplane that broke the
       record" [syn: hydrofoil, hydroplane]
    v 1: glide on the water in a hydroplane [syn: hydroplane,
         seaplane]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "hydroplane":
   aeroboat, aeroplane, airlift, airplane, amphibian, balloon,
   be airborne, boat seaplane, clipper, cruise, drift, ferry, flit,
   floatplane, fly, flying boat, glide, hop, hover, jet, navigate,
   sail, sailplane, seaplane, soar, take the air, take wing, volplane,
   waterplane, wing