The Collaborative International Dictionary of English v.0.48:

Undertow \Un"der*tow`\, n. (Naut.)
   The current that sets seaward near the bottom when waves are
   breaking upon the shore.
   [1913 Webster]

WordNet (r) 3.0 (2006):

undertow
    n 1: an inclination contrary to the strongest or prevailing
         feeling; "his account had a poignant undertow of regret"
    2: the seaward undercurrent created after waves have broken on
       the shore [syn: undertow, sea puss, sea-puss, sea
       purse, sea-purse, sea-poose]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "undertow":
   affluence, afflux, affluxion, concourse, confluence, conflux,
   coral heads, course, crosscurrent, current, defluxion, downflow,
   downpour, drift, driftage, flow, flowing, fluency, flux, gush,
   inflow, ironbound coast, ledges, lee shore, mill run, millrace,
   onrush, onward course, outflow, pitfall, quicksands, race,
   rockbound coast, rocks, run, rush, sandbank, sandbar, sands, set,
   shallows, shoals, spate, stream, surge, tide, trend, undercurrent,
   water flow