In A Christmas Carol Dickens uses the three ghosts to show Scrooge the development of his attitude to Christmas--which works as a broader symbol for humanity in general--the relations of his attitude to the attitude of [virtually] all other people in England and the eventual consequences of his attitude toward Christmas--a cold, unmourned and lonely death after which his pockets will be picked and his belongings sold off. When Scrooge magically travels to the lighthouse then to the ship, he is in the company of the second ghost, The Ghost of Christmas Present. The objective of the Ghost's choice of destinations is to show Scrooge scenes that are generally agreed to be some of the most desolate, lonely, dangerous, and despairing.
The life of lighthouse watchers is certainly lonely and isolated, and they see vast dangers when the seas roar and roil all around them while crashing ships and lives at their very feet. Surely such lighthouse keepers would share Scrooge's disdain for the fruit of Christmas, which is kindness, charitableness, and loving acceptance of life and the world. Yet what Scrooge sees is two men joining together in their remote and isolated refuge from storms and disaster by sharing a meal together in a spirit of rejoicing and thanks:
two men who watched the light had made a fire ... . Joining their horny hands over the rough table at which they sat, they wished each other Merry Christmas in their can of grog;
The Ghost takes Scrooge to the ship for the same reasons. The life of a ship's crew is fraught with danger, loneliness, isolation, and often the hostility of violent sailors’ tempers. Yet what Scrooge sees in men thinking kindly of Christmases past and of dear loved ones in families and among friends. He also sees them being glad to know they are lovingly and kindly thought of themselves by the dear ones who await them ashore:
had a kinder word for another on that day than on any day in the year; ... . and had remembered those he cared for at a distance, and had known that they delighted to remember him.
The surprise Scrooge gets is the sudden transition from the desolate raging sea, having Death as its guardian, to the sound of laughter: "it was a great surprise to Scrooge, ... to hear a hearty laugh." He is further surprised when he finds the laughter comes from his nephew who is safely secured in his own parlor surrounded by his loving wife and merry friends:
much greater surprise to Scrooge to recognise it as his own nephew's and to find himself in a bright, dry, gleaming room.
They happen to be laughing at Fred's amused rendering of the anecdote of his encounter with Scrooge during which Fred invites him to the festivities and Scrooge roughly declines with a "Bah! Humbug!" Scrooge is also surprised that they feel sorry for him and don't count his wealth as giving a benefit to him.
He said that Christmas was a humbug, as I live! ... His wealth is of no use to him. He don't do any good with it....
to show that people under the most unfortunate circumstances still celebrated the season.