In what region of England are the marshes of Great Expectations found?
Well these marshes must have been near the coast, but Dickens I think purposely never reveals exactly where these marshes are. We do know that it takes a good part of the day by coach to get to London. In chapter 20 it reports:
The journey from our town to the metropolis, was a journey of about 5 hours.
Once again this quote demonstrates the anonymity of Dickens as to the whereabouts of these marshes. Beyond this it does report this metropolis is London proper. So a range of places could be those marshes!
You could do some research into the locations of the prison ships of the Victorian era to see what part of the coast these might have been docked off of. The convict probably took a straight lined path to that shore when he escaped.
Although Charles Dickens does not actually state it in the novel, I have read in quite a few places that the marshes near the town where Pip grows up are in Kent.
Kent is the region off to the east of London. There were a number of prison hulks that were moored in Kent. Perhaps the best known of these were moored off the town of Medway in Kent. The town's website today claims that the hulks in Great Expectations were the hulks that were moored off Medway. I think this is supported by the part at the beginning of Chapter 20 where Pip's coach runs into traffic at Cheapside Cross which is, I believe, on the east side of London.