I would think that Dick Musgrove would have been educated in the basics before he left for the Navy. He may not have been very good at it, but it isn't likely that a schoolmaster on a ship would be teaching him to read and write, at least at a basic level, since he was from a family with money. I know that he is described as being good for nothing, but he must have known the basics.

It is my understanding that the Captain had a moral responsibility to look after the boys and midshipmen, especially since some boys went to sea before 14. He would hold on to their money for them, quiz them in navigation and naval-related skills, and look after their development in general. I would imagine that some captains did this task more diligently than others, just as they varied in other characteristics.