The title may declare the setting of this poem to be "London, 1802," but we don't envision a big, bustling, dirty city when we read it – instead, the mental space created by the poem is the "fen of stagnant waters" (2-3) that Wordsworth summons up to represent the state of the English character.
This is not a happy little pond; instead, it's gross and green and covered in algae, and it probably doesn't smell all that great. Imagine the poet disdainfully picking out a path around this mess, trying not to get mired in the mud – the foul mud of a depraved society, that is! Sorry, all this wilderness imagery is making us unusually dramatic.
Anyway, Wordsworth is unhappily squelching around in the metaphorical fen, thinking of how great England used to be. To continue with the whole body of water thing, the country and its people were previously clear, clean, and pure. In contrast to the current swampy and unhealthy state of the nation, Wordsworth creates another imaginary space, that of the open sea (Ah! Finally, a breath of fresh air.) and the clear, broad expanse of the sky, both of which represent the spiritual and poetic greatness of Milton, who in turn represents England's former splendor.