The antecedent for "It" in line nine of Wordsworth's "The World Is Too Much With Us" is, in general, nature.
Specifically, the "It" refers back to "The Sea" in line five and "The winds" in line six. But it also encapsulates the "Nature" of line three. The sea and the winds comprise nature in the poem together with, for example, the "pleasant lea" that appears later in the poem.
The use of the pronoun here provides unity to the poem. The pronoun recalls everything the speaker has said up to that point about nature. This pulls the octave together (an octave is usually the first eight lines of an Italian sonnet, but here it's the first 8.5) and in a sense summarizes it. The octave transitions then into the abrupt "--Great God!" of the sestet.