The Collaborative International Dictionary of English v.0.48:

Gunwale \Gun"wale\, n. [Gun + wale. So named because the upper
   guns were pointed from it.] (Naut.)
   The upper edge of a vessel's or boat's side; the uppermost
   wale of a ship (not including the bulwarks); or that piece of
   timber which reaches on either side from the quarter-deck to
   the forecastle, being the uppermost bend, which finishes the
   upper works of the hull. [Written also gunnel.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

gunwale
    n 1: wale at the top of the side of boat; topmost planking of a
         wooden vessel [syn: gunwale, gunnel, gun rest]