watchman vs watcher

watchman and watcher both are nouns.

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

As nouns, watchman and watcher are synonyms defined as:

Other synonyms of watchman include security guard.

watchman (noun)	watcher (noun)
a guard who keeps watch	a guard who keeps watch
	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