What is the point of Scrooge's visits to the lighthouse and the ship?
The spirit takes Scrooge to the lighthouse and the ship so that he can see people in rough situations who still have the Christmas spirit. The words used to describe these places (dismal, chafed, dashed, wild, solitary, black and heaving sea) are a sharp contrast to the spirit of the men they observe. They men are celebrating, enjoying the company of one another, revisiting old memories, thinking of loved ones, being kind to one another, and wishing one another a Merry Christmas.
Built upon a dismal reef of sunken rocks, some league or so from shore, on which the waters chafed and dashed, the wild year through, there stood a solitary lighthouse. Great heaps of sea-weed clung to its base, and storm-birds -- born of the wind one might suppose, as sea-weed of the water -- rose and fell about it, like the waves they skimmed.
But even here, two men who watched the light had made a fire, that through the loophole in the thick stone wall shed out a ray of brightness on the awful sea. Joining their horny hands over the rough table at which they sat, they wished each other Merry Christmas in their can of grog; and one of them: the elder, too, with his face all damaged and scarred with hard weather, as the figure-head of an old ship might be: struck up a sturdy song that was like a Gale in itself.
Again the Ghost sped on, above the black and heaving sea -- on, on -- until, being far away, as he told Scrooge, from any shore, they lighted on a ship. 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.