packet boat vs mailboat

packet boat and mailboat both are nouns.

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

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

Other synonyms of packet boat include mail boat, packet.

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