watcher vs viewer

watcher and viewer both are nouns.

Word	Noun	Adjective	Verb	Adverb
watcher	Yes	No	No	No
viewer	Yes	No	No	No

As nouns, watcher and viewer are synonyms defined as:

watcher and viewer: a close observer; someone who looks at something (such as an exhibition of some kind)

Other synonyms of watcher include looker, spectator, witness.

watcher (noun)	viewer (noun)
a guard who keeps watch	an optical device for viewing photographic transparencies
a close observer; someone who looks at something (such as an exhibition of some kind)	a close observer; someone who looks at something (such as an exhibition of some kind)
a person who keeps a devotional vigil by a sick bed or by a dead body