The Collaborative International Dictionary of English v.0.48:

Waterspout \Wa"ter*spout`\, n.
   A remarkable meteorological phenomenon, of the nature of a
   tornado or whirlwind, usually observed over the sea, but
   sometimes over the land.
   [1913 Webster]

   Note: Tall columns, apparently of cloud, and reaching from
         the sea to the clouds, are seen moving along, often
         several at once, sometimes straight and vertical, at
         other times inclined and tortuous, but always in rapid
         rotation. At their bases, the sea is violently agitated
         and heaped up with a leaping or boiling motion, water,
         at least in some cases, being actually carried up in
         considerable quantity, and scattered round from a great
         height, as solid bodies are by tornadoes on land. --Sir
         J. Herschel.
         [1913 Webster]

WordNet (r) 3.0 (2006):

waterspout
    n 1: a tornado passing over water and picking up a column of
         water and mist
    2: a heavy rain [syn: downpour, cloudburst, deluge,
       waterspout, torrent, pelter, soaker]
    3: a channel through which water is discharged (especially one
       used for drainage from the gutters of a roof)

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "waterspout":
   baguio, beak, brash, burst of rain, cloudburst, cyclone, deluge,
   downfall, downflow, downpour, downspout, drencher, dust devil,
   flood, gargoyle, gushing rain, heavy rain, plash, pour, rainburst,
   rainspout, rainstorm, rotary storm, sand column, sandspout, scud,
   soaker, soaking rain, spate, spout, tornado, torrent of rain,
   twister, typhoon, whirlblast, whirlwind, wind eddy