The speaker sets two worlds against each other: the wild, unruly sea and a quiet, enclosed life among stone. The sea is described as "her own matter," which suggests something that belongs wholly to itself, rough and raw and not easily shared or tamed. Against that, the speaker longs for "an hour in stone," a small, still space where people "tend to a thin world." That phrase is a little odd and open. It might mean a life pared down and modest, or a place where things feel delicate and slight.
The last lines bring the poem down to something ordinary and almost funny: the speaker's "chief joy" is buying a "virginal book," meaning one that is new and unread. The mood is wistful but gently playful. The speaker admires the sea's power yet prefers shelter, and finds happiness in the small pleasure of a fresh book, full of unopened possibility.