sea-duty vs duty assignment

sea-duty and duty assignment both are nouns.

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

As nouns, duty assignment is a hypernym of sea-duty; that is, duty assignment is a word with a broader meaning than sea-duty:

sea-duty: naval service aboard a ship at sea
duty assignment: a duty that you are assigned to perform (especially in the armed forces)

Other hypernyms of sea-duty include assignment.

sea-duty (noun)	duty assignment (noun)
naval service aboard a ship at sea	a duty that you are assigned to perform (especially in the armed forces)