Describe the cabin-boy. Why has he come to Shaws?

The cabin-boy is described as "a half-grown boys in sea clothes," "blue with cold", with a "look between tears and laughter." The boy came to the Shaws with a letter.

Stay, brother!’ he cried. ‘Have you no fun about you? or do you want to get me thrashed? I’ve brought a letter from old Heasyoasy to Mr. Belflower.’ He showed me a letter as he spoke.