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]