The taste of seawater is a dead giveaway: too much salt can damage the human body.
Remember that seawater is 3.5% or (35 g/L, or 599 mM) salt or dissolved minerals ((salinity measurement is a total of all the salts that are dissolved in the water. Although 35 parts per thousand is not very concentrated (the same as 3.5 parts per hundred, o/o, or percent) the water in the oceans tastes very salty.Source)). If you drink seawater in a desperate attempt to survive out in the ocean you would just do more harm to your body and increase your chances of dying instead of living long enough until rescue arrives.
This would lead to Hypernatremia (hyper= too much + natr= sodium + emia=in the blood) a condition of having too much salt and too little water in your body ((This water loss can occur from illnesses with vomiting or diarrhea, excessive sweating from exercise or fever, or from drinking fluid that has too high concentrations of salt. Source.)). Simply because your body will work extra hard to get rid of the increased excess salt. This process requires water and since your body is the only source of water with lower salt concentrations, you would actually need more water to survive. It would make your situation much worse than before.
Dehydration would worsen and your chances of making it out alive is greatly decreased. That’s one of the perils of being lost at sea, you’re surrounded by water and yet not a single drop is readily safe for drinking.
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