The Collaborative International Dictionary of English v.0.48:

Debater \De*bat"er\, n.
   One who debates; one given to argument; a disputant; a
   controvertist.
   [1913 Webster]

         Debate where leisure serves with dull debaters. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

debater
    n 1: someone who engages in debate [syn: debater, arguer]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "debater":
   Philadelphia lawyer, apologist, arguer, argufier, casuist,
   controversialist, declaimer, demagogue, disceptator, disputant,
   guardhouse lawyer, haranguer, jawsmith, logomacher, logomachist,
   mooter, panelist, pilpulist, polemic, polemicist, polemist,
   public speaker, rabble-rouser, ranter, speaker, speecher,
   speechifier, speechmaker, spieler, spokesman, spokeswoman, spouter,
   talker, tub-thumper, wrangler