Certainly, the phrase "fen of stagnant waters" carries quite a negative connotation. A fen is an area that is mostly or wholly covered with water, where water can pool and sort of go rancid and stinky with rotting plant matter and pungent soil. If something is stagnant, then it is unchanging or stale ("constant" or "still" would be words that mean something similar but have more positive connotations). To call England a stagnant fen is to say that it has not developed appropriately, that it is not evolving or growing; instead, this country with a history of heroism and legend and honor has grown stale and rotten and corrupt, as water does when it stagnates and stands for a long period of time. Just as water would get scummy and putrid, so have the morals of England. These negative connotations and images absolutely inform the reader's understanding of the speaker's view of English society in 1802.