What songs are being hummed by the ship's officers at sea?

From the text:

They stood beside the helmsman at the wheel, the look-out in the bow, the officers who had the watch; dark, ghostly figures in their several stations; but every man among them hummed a Christmas tune, or had a Christmas thought, or spoke below his breath to his companion of some bygone Christmas Day, with homeward hopes belonging to it. And every man on board, waking or sleeping, good or bad, had had a kinder word for another on that day than on any day in the year; and had shared to some extent in its festivities; and had remembered those he cared for at a distance, and had known that they delighted to remember him.