The way I would summarize Chapter 5 is by saying that this is the chapter where Harvey Cheyne really becomes part of the crew and a real fisherman.
During this chapter, we see all sorts of things that show Harvey learning the ropes and that show him getting to know the rest of his crew better. For example, we see him learning how to do a bit of navigation. And we see him learning the lore of the old time navy from Tom Platt.
During the chapter, Harvey learns a lot about fishing and about being a member of a fishing boat's crew.