Drinking seawater is highly dangerous and can be fatal. The high concentration of dissolved salts makes the water counterproductive for hydration, intensifying thirst and accelerating dehydration. Consuming this water introduces an excess of sodium that the kidneys cannot process without drawing upon the body’s existing water reserves. In a survival situation, drinking seawater shortens the time to death.
The Core Problem: Extreme Salinity Levels
The primary issue with seawater is its extreme salt concentration, which is far beyond what the human body can safely manage. Ocean water typically contains about 3.5% dissolved salts by mass, meaning every liter holds roughly 35 grams of sodium chloride and other minerals. In contrast, the human body maintains a tightly regulated internal salinity, with blood plasma being approximately 0.9% salt. This difference means that seawater is about four times saltier than the internal fluids the body requires to function normally.
The Body’s Response to Excess Salt
The mechanism that renders seawater toxic is based on osmosis, the movement of water across a semipermeable membrane to balance solute concentrations. When highly saline water enters the bloodstream, it dramatically increases the concentration of salt in the blood, known as hypernatremia. The kidneys are responsible for regulating sodium levels and must filter and excrete this massive salt overload.
However, the human kidney can only produce urine with a maximum salt concentration of roughly 1,200 milliosmoles per liter. Since seawater is more concentrated than the maximum output of the kidney, the body must use a greater volume of fresh water to dilute the salt for excretion than the amount of water consumed. For every liter of seawater ingested, a person must urinate more than a liter of body water to flush out the excess sodium. This process forces the kidneys to pull water from the body’s cells and tissues. The net result is a negative water balance, where drinking the water paradoxically causes a greater overall loss of hydration.
Acute Risks: Severe Dehydration and Illness
The counterproductive effect on the kidneys quickly leads to rapid and severe hypertonic dehydration. As water is diverted from cells to dilute the salt, tissues shrink, and the body’s systems begin to fail. Immediate symptoms often include intense, unquenchable thirst, followed by vomiting and diarrhea as the digestive system attempts to expel the high salt content. This gastrointestinal distress further accelerates fluid loss.
As the condition worsens, the buildup of sodium in the blood begins to affect the central nervous system. This can manifest as confusion, hallucinations, and delirium, impairing judgment. If consumption continues, the severe dehydration and electrolyte imbalance can lead to muscle spasms, seizures, and ultimately organ failure. Analysis of historical life raft incidents has estimated a significantly higher risk of death for those who drank seawater compared to those who abstained.
Secondary Risks: Microbial and Chemical Contamination
Beyond the issue of salinity, seawater presents numerous other health hazards related to contamination. Ocean water is often a reservoir for various pathogens that can cause acute gastrointestinal illness. These include bacteria such as Escherichia coli, Salmonella, and Vibrio species, which can cause severe diarrhea and potentially life-threatening infections. Viruses and parasites from sewage runoff are also frequently present.
Furthermore, coastal waters suffer from chemical pollution from industrial and agricultural runoff. This includes heavy metals like mercury and lead, pesticides, and petroleum products. These contaminants, along with widespread microplastics, present a significant long-term health risk. Ingesting these substances can lead to immediate poisoning or the bioaccumulation of toxic compounds in the body over time.