How is Magwitch captured? What happens to Compeyson?

They get up the next morning and start rowing down the river, picking up Magwitch at the preappointed time. They row downstream all day and put in on shore at an inn for the night.

They start off the next day and are within a few feet of a steamer that they hope to board when another boat pulls alongside to stop them. In the confusion, Pip sees Compeyson leading the other boat, but the steamer is on top of them. The steamer crushes Pip's boat, Compeyson and Magwitch disappear under the water, and Pip, Startop and Herbert find themselves in a police boat of sorts.

Magwitch finally comes up from the water. He and Compeyson and wrestled for a while, but Magwitch let him go and now Compeyson is presumably drowned. Once again, Magwitch is shackled and arrested.

Pip sits down next to the injured and exhausted Magwitch, and feels that he will stay by Magwitch's side until the end. Pip also realizes that the English government will take all of Magwitch's fortune.