port watcher vs security guard

port watcher and security guard both are nouns.

Word	Noun	Adjective	Verb	Adverb
port watcher	Yes	No	No	No
security guard	Yes	No	No	No

As nouns, security guard is a hypernym of port watcher; that is, security guard is a word with a broader meaning than port watcher:

Other hypernyms of port watcher include watcher, watchman.

port watcher (noun)	security guard (noun)
a watchman on a wharf	a guard who keeps watch