The Collaborative International Dictionary of English v.0.48:

Shipmate \Ship"mate`\, n.
   One who serves on board of the same ship with another; a
   fellow sailor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

shipmate
    n 1: an associate on the same ship with you

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "shipmate":
   ace, amigo, associate, bedfellow, bedmate, birthmate, bosom buddy,
   buddy, bunkie, bunkmate, butty, camarade, chamberfellow, chum,
   classmate, clubmate, colleague, comate, companion, company,
   compeer, comrade, confrere, consociate, consort, copartner,
   copemate, copesmate, couchmate, cradlemate, crony, cupmate, fellow,
   fellow student, general partner, girl friend, gossip, jailmate,
   mate, messmate, old crony, pal, pard, pardner, partner, pewmate,
   playfellow, playmate, roommate, schoolfellow, schoolmate,
   secret partner, shelfmate, shopmate, side partner, sidekick,
   sidekicker, silent partner, sleeping partner, special partner,
   tablemate, teammate, tentmate, watchmate, waymate, workfellow,
   yokefellow, yokemate