Little we see in Nature that is ours (3)
People have become so obsessed with "getting and spending" that they have lost the ability to see nature as somehow relevant to their humanity. The language of quantity ("little") and possession ("ours") suggests one reason for this breach: our approach to nature has been infected with a consumerist ideology obsessed with getting, spending, measuring, etc.
We have given our hearts away, a sordid boon! (4)
The speaker suggests a connection between our "hearts" and nature. There is a sense in which people have "given" nature "away" as well because they no longer feel any connection to it, aren't moved by it, etc. To give nature away is to give away an essential part of our humanity.
This sea that bares her bosom to the moon; (5)
The speaker describes a sea he is viewing as something that should move us; the sea is compared to a woman bearing her bosom to the moon, almost as if the sea were nursing the moon. The image of nursing comes up again when the poet says he would rather be a "pagan suckled in a creed outworn."