Where did the phrase "our ship has come in" originate?

In the nineteenth century in Bristol, England, sailors' wives would tell merchants who were extending credit to them that they would pay their bills when their men returned from a sea voyage or "when our ship comes in." Today the phrase is usually a promise to pay someone or buy something when ones' finances improve.

From the days of sea trade in wooden sailing ships. "The Merchant Of Venice" has this as its main theme incidentally.

It is an old merchant marine term. You used to finance a crew to sail off with goods and come back with goods. There were handsome profits in foreign "exotic" goods such as tea and silk. However, the risks were high with storms, crooked captains, pirates, etc... So when "your ship has come in" the risk and uncertainty was gone and you were anticipating the money you will make off of the cargo.

https://idioms.thefreedictionary.com/waiting+for+my+ship+to+come+in