The Collaborative International Dictionary of English v.0.48:

Hydro-aeroplane \Hy"dro-a"["e]r*o*plane`\ (A["e]ronautics)
   An a["e]roplane with a boatlike or other understructure that
   enables it to travel on, or to rise from the surface of, a
   body of water by its own motive power; -- now usually
   referred to as a seaplane. [obsolescent]
   [Webster 1913 Suppl. +PJC]