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]