Pratt uses the chambered nautilus to talk about how people grow and change. As the creature gets bigger, it seals off the chambers it has outgrown, but those empty rooms stay in its spiral shell, a kind of record of where it has been. The speaker points out that this growing shell isn't a weight on the animal. The nautilus uses it to float, and when the moment comes, to dive.
In the last stanza the poem turns to "we." Everything we have known and believed, even the ideas we've moved past, can help us find our way through "the sea we're in." The tone is calm, steady, and gently encouraging. The speaker doesn't seem to regret outgrown beliefs or treat them as mistakes. Instead, the poem suggests that changing your mind is natural, and that your past stays with you as support rather than baggage. The simple, even rhythm and the closing phrase "as long as we're alive" give it a quiet, reassuring finish.