(n.) One who serves on board of the same ship with another; a fellow sailor.

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