sea-duty vs service abroad

sea-duty and service abroad both are nouns.

Word	Noun	Adjective	Verb	Adverb
sea-duty	Yes	No	No	No
service abroad	Yes	No	No	No

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

Other synonyms of sea-duty include shipboard duty.

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