At the beginning of the book, Pip is a poor orphan growing up in a town near the marshes. The marshes are often misty and seem very mysterious. This is connected to what they represent.
In my opinion, the marshes represent danger to Pip in this book. You can see this most clearly at the beginning of the book. At that point, when he goes out into the marshes he runs into the convicts who seem to be a threat to his life.
Later on in the book, he has to travel through the marshes to London to become a gentleman. Becoming a gentleman isn't a physical danger to him, but it is a danger to his morality and his soul.
I'd say the marshes represent danger.
I would say the marshes can also represent emptiness. Pip is first at the marshes studying the names of his family members who have passed away. He does so with a stick poking at their graves. When the life of the convict comes in contact with Pip, Pip finds the desparation of the covict almost immediately. When people are in that desparate state, they are empty, waiting to be filled.
The convict's first need is hunger and he looks to be filled by Pip. Out of fear Pip does help the convict, but I think it's more than that. Pip has an empty life. This convict at least provided some action for a time.
The life Pip leads in this small town feels empty to him. This is why the book (among other reasons) is entitled Great Expectations. Pip wants something more and this location at the marshes represents the opposite of more, nothingness or emptiness.
In regards to the socioeconomic elements of "Great Expectations" I'd say the marshes may represent lower life forms feeding higher life forms. It is the death and decay that allows the plants and life of the marshes to thrive. Similarly it is the undervalued labor of the lower classes that support the lifestyles of the upper class.
I believe that the marshes represent the hardships through Pips childhood. Because thats where he was threatened by a convict (Magwitch) and then again by another convict (Compeyson).