sea-duty vs shipboard duty

sea-duty and shipboard duty both are nouns.

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

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

Other synonyms of sea-duty include service abroad.

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