Pretty standard operating for the British to throw POWs and criminals into floating prison ships. Remember Pip running into Magwitch escaping from a prison hulk at the beginning of Dickens Great Expectations.
That first chapter of “Great Expectations” was so full of plot holes I’m still scratching my head...
Wonderful description of the former fen country but who was the fellow convict Magwitch enlisted Pip to aid who then ran down Magwitch and got them both recaptured? Or something like that?