When stranded, the presence of water on all sides can create a deceptive sense of security. Although surrounded by the vast ocean, the human body cannot process the high salt content of seawater. Finding a source of safe, potable water quickly becomes the most immediate challenge to survival. The average person can survive only about three days without water, making the ability to convert or locate fresh water the most important skill in this scenario.
The Physiological Danger of Seawater Consumption
Drinking raw ocean water actively works against the body’s efforts to maintain hydration. Seawater contains an average salt concentration of about 3.5% sodium chloride and other minerals, which is significantly higher than the salt level found in human blood. Ingesting this highly saline fluid leads to hypernatremia, an excessive concentration of sodium in the blood.
The kidneys are responsible for filtering waste and regulating the balance of salts and water. To eliminate the sodium load from seawater, the kidneys must produce urine that is equally or more concentrated than the ingested fluid. However, the human kidney cannot produce urine with a salt concentration greater than about 2.2%.
Because the kidneys cannot handle the high salinity, they are forced to pull fresh water from the body’s existing reserves to dilute and flush out the excess salt. This process dramatically increases the body’s net water loss, accelerating dehydration. The high sodium level in the bloodstream draws water out of the body’s cells via osmotic pressure, causing cellular dehydration and shrinkage. Continued strain on the kidneys can lead to acute kidney injury and eventual organ failure.
Emergency Desalination Techniques
Since direct consumption of seawater is dangerous, the primary survival strategy involves desalination, the process of removing salt. This is achieved through distillation, which uses the principle that water evaporates at a much lower temperature than salt. The salt remains behind while the water turns into vapor, which is then collected as pure, fresh water.
Solar Still Construction
One effective method is constructing a solar still, requiring a heat source, a container, and a way to capture condensation. To create a simple pit still, dig a hole and place a cup or container in the center. Pour seawater into the pit, avoiding the collection cup, and cover the entire hole with a clear plastic sheet or tarp. Place a small weight, like a rock, in the center of the plastic sheet directly above the container to create a funnel shape.
The sun heats the still, causing the water to evaporate into vapor. This vapor rises until it hits the cooler underside of the plastic sheet, where it condenses into liquid droplets. Gravity causes these pure water droplets to run down the angled plastic and drip directly into the container below. A well-made still can yield a few hundred milliliters of potable water over a day, but the process is slow.
Boiling and Condensation
Boiling and condensation is a faster form of distillation if a heat source and two containers are available. Seawater is boiled in a sealed container, and the resulting steam must be captured. The steam is pure water vapor, leaving the salt residue behind in the boiling container. Angle a second, cooler container or plastic sheeting over the steam’s path. As the steam contacts the cool surface, it condenses and can be directed to drip into a separate collection vessel. Simply boiling the water until it evaporates will not work, as the salt will only become more concentrated in the remaining liquid.
Alternative Coastal Water Sources
When desalination resources are unavailable, the focus must shift to locating naturally occurring fresh water. Rainwater is the purest and safest source of water in a survival situation because it is naturally distilled. Any wide, non-porous material, such as a tarp, plastic sheet, or large leaves, can be used to funnel and collect rainfall into a container.
Dew collection is a viable option in coastal environments where humidity is often high. An absorbent cloth or sponge can be wiped across surfaces like boat hulls, smooth rocks, or large leaves early in the morning before the sun dries the moisture. The collected water can then be wrung out into a drinking vessel. This method is slow but provides a small, steady source of moisture.
Along a shoreline, look for freshwater seeps or streams running into the ocean. Following a stream uphill, away from the tidal zone, can lead to a reliable source of fresh water, though it should still be purified to remove biological contaminants. In vegetated areas, water can also be collected from plants through transpiration by tying a clear plastic bag securely around a cluster of non-poisonous leaves.
Coconuts and Groundwater
Certain coastal plants, such as young, green coconuts, provide a sterile liquid that is a good source of hydration. However, the milk from mature, brown coconuts contains high levels of fat and can act as a diuretic, potentially counteracting hydration efforts. As a last resort, digging a shallow well in the sand just above the high-tide line may yield brackish groundwater. While this water is slightly less salty than ocean water, it still carries risks and should be purified.