Two closely related themes in the early chapters are the primordial nature of the oceans in supporting all life and their importance in the development of our species and its predecessors.

The author explains both the current (as of its 1951 publication) state of knowledge about the ocean's surface and depths as well as some significant earlier phases in understanding these phenomena. While she is primarily concerning with the surface of the oceans, she also includes significant topics about deep-water oceanography and ocean-floor topography. The underwater aspects are emphasized in their roles creating new lands through volcanic activity.

Looking at the surface in relation to the atmosphere, Carson tells us how currents and waves have influenced human activity. The ways that events far out at sea effect life on shore is explored, including such extreme phenomena as tsunamis. On a more routine basis, however, Carson emphasizes, the daily workings of the tides have enabled human subsistence on the shores and contributed to our development of marine resources. The ocean's role in regulating climate onshore is stressed as well.

The last section is especially concerned with human exploration and sea-going voyages. Although Carson does not aim to explore the technological history of ships, she helps the reader understand the essential underlying role of changing scientific knowledge in enabling that technology. This is underscored in the final chapter on the history of navigation.

One important theme is the actual and potential damage inflicted by human beings in our age-old quest to co-exist with and conquer the waters.

As the book is intended for a popular audience, rather than written as an academic treatise, two related themes are stewardship and advocacy: the possible and recommended roles that people can play to help protect the oceans.

To wrap up the book, Carson returns to the theme of human impact as she reminds us of our vital role in protecting the nurturing waters.