The Collaborative International Dictionary of English v.0.48:

Watcher \Watch"er\ (-[~e]r), n.
   One who watches; one who sits up or continues; a diligent
   observer; specifically, one who attends upon the sick during
   the night.
   [1913 Webster]

WordNet (r) 3.0 (2006):

watcher
    n 1: a close observer; someone who looks at something (such as
         an exhibition of some kind); "the spectators applauded the
         performance"; "television viewers"; "sky watchers
         discovered a new star" [syn: spectator, witness,
         viewer, watcher, looker]
    2: a guard who keeps watch [syn: watchman, watcher,
       security guard]
    3: a person who keeps a devotional vigil by a sick bed or by a
       dead body

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "watcher":
   Argus, Charley, TV-viewer, airplane spotter, beholder,
   bird-watcher, bystander, drugstore cowboy, eyewitness,
   fire patrolman, fire warden, fireguard, forward observer, gaper,
   gazer, gazer-on, girl-watcher, goggler, kibitzer, looker,
   looker-on, lookout, lookout man, night watchman, observer, ogler,
   onlooker, patrol, patroller, patrolman, perceiver, percipient,
   picket, roundsman, scout, seer, sentinel, sentry,
   sidewalk superintendent, spectator, spectatress, spectatrix,
   spotter, televiewer, television-viewer, vedette, video-gazer,
   viewer, watch, watchkeeper, watchman, witness