Imagine yourself in a ship out on the high seas. It stands to reason that you would want smooth sailing for that ship. You'd want everything about the vessel to be sound and secure, to function properly and fare well.
But what if you were on the boat involuntarily? What if it were one of the infamous eighteenth-century slave ships? Could you possibly pray for the welfare of such a despicable voyage?
Despicable or not, if you're on board, you pray for smooth sailing. And such was the logic of God's word to the exiles of Jeremiah's day.