packet boat vs mail boat

packet boat and mail boat both are nouns.

Word	Noun	Adjective	Verb	Adverb
packet boat	Yes	No	No	No
mail boat	Yes	No	No	No

As nouns, packet boat and mail boat are synonyms defined as:

Other synonyms of packet boat include mailboat, packet.

packet boat (noun)	mail boat (noun)
a boat for carrying mail	a boat for carrying mail