shipboard duty vs sea-duty

shipboard duty and sea-duty both are nouns.

Word	Noun	Adjective	Verb	Adverb
shipboard duty	Yes	No	No	No
sea-duty	Yes	No	No	No

As nouns, shipboard duty and sea-duty are synonyms defined as:

Other synonyms of shipboard duty include service abroad.

shipboard duty (noun)	sea-duty (noun)
naval service aboard a ship at sea	naval service aboard a ship at sea