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Anyone who has played in the ocean knows getting a little sea water in your mouth and swallowing it will not kill you.

Seawater contains salt. When humans drink seawater, their cells are thus taking in water and salt. While humans can safely ingest small amounts of salt, the salt content in seawater is much higher than what can be processed by the human body. Additionally, when we consume salt as part of our daily diets, we also drink liquids, which help to dilute the salt and keep it at a healthy level. Living cells do depend on sodium chloride (salt) to maintain the body’s chemical balances and reactions; however, too much sodium can be deadly.

Human kidneys can only make urine that is less salty than salt water. Therefore, to get rid of all the excess salt taken in by drinking seawater, you have to urinate more water than you drank. Eventually, you die of dehydration even as you become thirstier.

Source NOAA

If I am thirsty and I drink sea water, it will make things worse. During normal activities if you have taken in some sea water, you will naturally consume enough fresh water (or fluids) to flush excessive salt out.

Maybe the real question is; if you drink sea water, how much fresh water do you need to drink to off set the sea water you drank? For every 1 unit of sea water, how many units of fresh water do I need to act as an antidote?

As pointed out in answers to the question How safe is drinking distilled water? it takes a lot of energy to make fresh water from sea water. Assume you are in a survival situation, with only sea water available, one of your companions has consumed a quantity (1 unit) of sea water.

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James Jenkins
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How much sea water can I safely drink? = None

If you drink sea water, how much fresh water do you need to drink to off set the sea water you drank? = 2.8 units of distilled water per 1 unit of sea water (to neutralize without adding hydration)

The "scientific" answer to this question involves a lot of complex math, human physiology and significant variables. In its simplest form the question has one concept and two factors

Concept

Tonicity: in the same way that water in a container always wants to be at the same level (flat) it also wants to contain the same amount of salt. Humans are "Bags of Mostly Water" that need to keep a specific ratio of salt to water, when the ratio of salt to water becomes too great the body dies of dehydration.

Factors

Normal Saline is a medical grade mixture of salt and water. It contains essentially the same amount of salt and water as a normally hydrated human. If you drink this it will not increase or decrease your hydration (you won't get more or less thirsty). It contains 9 grams of salt (NaCl) per liter of water.

Salinity of seawater:, there are a lot of variables, but on average seawater has a salinity of about 35 grams per liter of water.

Conclusion

To keep the balance of added salt and added water to a human body. You need to add them in the in the ratios as they are present, 9 grams per liter.

Seawater has 35 grams per liter of salt, if we mix one liter of distilled (no salt) water with one liter of seawater, we have 17.5 grams of salt for each liter of water present. When you do the math (3.5 / 0.9 = 3.8) it turns out you need a total 3.8 liters of water to get the same ratio of salt to water. So you need to add 2.8 litters of distilled water to your 1 liter of seawater to be neutral (isotonic).

It ain't simple

There are a lot of variables, and even some controversy. In the end it is undeniable that if obtaining fresh water is difficult, drinking some seawater is not going to make things better. If you are in survival situation, there is no "safe amount" of seawater you can drink.

Sweat

Per a comment about salt loss with sweating, I did some research. You lose about 900mg of salt per liter of sweat. Gatorade restores about 450mg of salt per liter. Sea water has 35 GRAMS of salt per liter that is 35,000mg. Drinking sea water, because you are sweating while lost at sea, is not a good idea.

Disclaimer

This answer is based on compiled research, most of the sources listed are Wikipedia. I did not find a reputable single source that provided a direct answer. While I believe the conclusions are accurate, there is room for error. Don't do any stupid experiments based solely on this answer!

James Jenkins
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    +1 for the nicest layman explanation of hydration and water tonicity (namely that you won't get less thirsty by drinking medical salinity water). Btw I'll do only smart experiments based on this answer! – Pavel Mar 11 '16 at 15:17
  • Working from very old memory here, but in the book Kon-Tiki they said they found it helpful to add a small amount of sea water to their fresh water supplies, that it extended them without creating negative effects. I don't recall how much they added, or if the book even said, or if they were actually just wrong and didn't realize it. Just an anecdote. – Carey Gregory Sep 18 '16 at 01:03
  • @CareyGregory Adding a small amount to fresh water very well may be in line with this advise since he is saying that adding 2.8 liters of fresh water to one liter of seawater brings the mixture to isotonic. Of course if you are sweating out more salt than you are intaking then you would want to adjust your mixture to compensate. However that is hardly a common problem, and is beyond the scope of this answer. – Erik Jan 17 '17 at 18:43
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    I've just asked Ways to desalinate water when “lost at sea”? – uhoh Sep 25 '18 at 04:37
  • If you're sweating a lot your body will lose a fair bit of salt (the concentration in sweat isn't well controlled like it is in urine), so a small proportion of seawater may be necessary or beneficial, depending on how much salt you're getting from food. Hyponatraemia isn't good – Chris H Sep 25 '18 at 11:34
  • @ChrisH Grams vs MG. you lose about 900mg of salt per liter of sweat. Gatorade restores about 450mg of salt per liter. Sea water has 35 GRAMS of salt per liter that is 35,000mg. – James Jenkins Sep 25 '18 at 12:46
  • @JamesJenkins go for it. I've read up a bit on Exercise-induced hyponatraemia (Wikipedia, see ref 3 especially) but in the context of endurance cycling with access to water and nutrition (12 hours/200km in up to 28C with little shade makes it start to appear relevant). A survival/seawater question would be quite different and interesting – Chris H Sep 25 '18 at 12:49
  • @JamesJenkins so in that case -- I estimate I lost ~6 or 7 litres in sweat that day, based on intake and (lack of) toilet stops -- I'd need 200ml of seawater to replace the salt. A small but not insignificant amount. I had some electrolyte drink tablets with me, and a salty lunch stop – Chris H Sep 25 '18 at 12:52
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    @ChrisH I just edited my answer to address the point. – James Jenkins Sep 25 '18 at 12:54
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    Adding something like a teaspoon to a tablespoon per litre (~1%) in the absence of other sources would be harmless to beneficial on that basis, but doesn't exactly increase your water supply, so your conclusion fits the context of this question perfectly. – Chris H Sep 25 '18 at 12:59
  • @JamesJenkins, an infusion of normal saline (0.9% NaCl) is used as a treatment for severe dehydration (distilled water would cause hyponatremia) Emedicine. So, normal saline does hydrate you. It can also hydrate you if you drink it, but some people would not be able to do so, some would vomit afterwards and some could get diarrhea, making dehydration worse. Anyway, in scenario with no fresh water but several bags of medical saline available, it would hydrate you if you manage to drink it and keep it in. – Jan Nov 13 '18 at 12:14
  • @Jan There are many variables Lactated Ringer and 0.45% NaCl (Half NS) are also used to treat severe dehydration with IV therapy. But those topics are beyond the scope of this answer. Note that section of the answer you are referencing talks about a "normally hydrated human", not a someone in severe dehydration. – James Jenkins Nov 13 '18 at 12:36
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    I agree that a normally hydrated person, after drinking 0.9% saline will urinate out all the amount drunk in a short time. But this is also true for plain water. What triggered me to comment was: "If you drink this [0.9% saline] it will not increase or decrease your hydration." This claim is within the answer that tries to explain why seawater is not good to drink. But 0.9% is OK, because it does hydrate you. It depends who/how reads the answer. You can take it as a picky comment and ignore it, as it suits you. For me, the paragraph about saline is just not necessary. – Jan Nov 13 '18 at 12:57
  • Using 0.9% saline in a calculation that gives you 2.8 L of water may not be appropriate. Guyton's physilogy (the text in grey on the right) estimates drinking 1 L of seawater results in excretion of 1.5 L of urine. So, you need 1.5 L of plain water to offset 1 L of seawater. I agree with your answer as a whole, though. – Jan Nov 13 '18 at 13:26