Water That Makes Men Mad
When Samuel Taylor Coleridge wrote the words, “Water, water everywhere, but not a drop to drink” in The Rime of the Ancient Mariner in 1798, the dangers of drinking seawater had been known for thousands of years. Seawater does indeed make men mad. Historical evidence indicates the ancient Egyptians knew seawater was not potable, but the earliest realization that it was unsafe to drink has been lost to antiquity. In pre-Columbian times the greatest fear of venturing too far from land on the ocean was not falling off the surface of the Earth but lack of fresh drinking water. From a human perspective the oceans, which cover 70% of Earth's surface, are still the most extensive and unique desert wildernesses on the planet. Saltwater constitutes 97% of Earth's water, and of the 3% that is fresh, two-thirds is frozen in glaciers and polar ice. Thus, a mere 1% of all the water on the planet (in lakes and rivers, groundwater, and the atmosphere) is fresh and can be used by terrestrial plants and animals.
For a mariner adrift at sea, the most common causes of death are hypothermia and dehydration, and the castaway's fate depends to a great extent on water temperature. Hypothermia is the survival-limiting problem more than 30° N or S of the equator; this safe temperature zone shrinks to 20° N or S in winter. Dehydration is the major problem on the oceans between the tropics of Cancer and Capricorn and in lifeboats and rafts is an important cause of death at all temperate latitudes.