The Collaborative International Dictionary of English v.0.48:

Whirlpool \Whirl"pool`\, n.
   [1913 Webster]
   1. An eddy or vortex of water; a place in a body of water
      where the water moves round in a circle so as to produce a
      depression or cavity in the center, into which floating
      objects may be drawn; any body of water having a more or
      less circular motion caused by its flowing in an irregular
      channel, by the coming together of opposing currents, or
      the like.
      [1913 Webster]

   2. A sea monster of the whale kind. [Obs.] --Spenser.
      [1913 Webster]

            The Indian Sea breedeth the most and the biggest
            fishes that are; among which the whales and
            whirlpools, called "balaenae," take up in length as
            much as four . . . arpents of land.   --Holland.
      [1913 Webster]

WordNet (r) 3.0 (2006):

whirlpool
    n 1: a powerful circular current of water (usually the result of
         conflicting tides) [syn: whirlpool, vortex,
         maelstrom]
    v 1: flow in a circular current, of liquids [syn: eddy,
         purl, whirlpool, swirl, whirl]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "whirlpool":
   Charybdis, Maelstrom, ado, back stream, backflow, backwash,
   backwater, bustle, centrifugate, centrifuge, countercurrent,
   counterflow, counterflux, dizzy round, eddy, flurry, furore, fuss,
   gulf, gurge, gyre, maelstrom, pirouette, pother, purl, rat race,
   reel, refluence, reflux, regurgitation, round, spin, surge, swirl,
   turn, twirl, vortex, wheel, whirl, whirligig, whirlwind, whorl