I Saw Three Ships / Song of the Ship
Hard to believe, but these are two completely different carols. I discovered the German "Song of the Ship" in the Oxford Carol Book, and immediately liked the additional depth its lyrics lent to a rather simplistic story. But then I stumbled upon how well they worked together, as call and answer... I wonder if the later English "Three Ships" composer had ever thought of it?

mp3 sound clip here, about 1/3 way down page.