bo'sun vs boatswain

bo'sun and boatswain both are nouns.

Word	Noun	Adjective	Verb	Adverb
bo'sun	Yes	No	No	No
boatswain	Yes	No	No	No

As nouns, bo'sun and boatswain are synonyms defined as:

bo'sun and boatswain: a petty officer on a merchant ship who controls the work of other seamen

Other synonyms of bo'sun include bo's'n, bos'n, bosun.

bo'sun (noun)	boatswain (noun)
a petty officer on a merchant ship who controls the work of other seamen	a petty officer on a merchant ship who controls the work of other seamen