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Ocean water has about 35 Grams of salt per liter. For the human body salt content in body fluids is about 9 Grams per liter. See related question How much sea water can I safely drink?

While it is best that your fresh water only contain very trace amounts of salt, water that is equal or less than 9 grams of salt per liter can help maintain your hydration level (trying to simplify a complex concept)

Many desert animals get all most of their hydration from food

If stranded at sea, and I eat a lot of fish flesh. Will it have a low enough salt content to help keep me hydrated?

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Let's say you lose 2 liters of water per day. To replace 2 liters of water by eating fish, you need to eat, for example, 2,500 g of cooked Pacific cod, which, besides water, also contains 500 g of protein and 3.4 g of sodium.

Consuming 3.4 g of sodium (8.5 g of salt) per day is not that much; some people consume it on a daily basis. I calculated (from here) that this amount of salt can result in about 240 mL of water loss by urine.

The bigger concern is protein, which is broken down into urea, which is excreted through the kidneys and drags some water with it in a similar way as sodium does.

1 gram of protein yields the amount of urea that needs 8 mL of urine to be excreted (see the source below). Consuming 500 g of protein from fish would therefore result in 4 liters of urine. So, consuming 2 liters of water by eating 2,500 g of fish leaves you with 2 liters of negative water balance solely due to urea.

So, if my calculation is roughly correct, eating only fish will make you even more dehydrated than eating and drinking nothing.

Source: NAP.edu

Urea, a major end product of metabolism of dietary proteins and amino acids, requires water for excretion by the kidneys. Renal excretion of 1 g of urea nitrogen (2.2 g of urea) requires 40 to 60 mL of water. Thus, if a person consumes 63 g of protein the volume of water required increases by 0.4 to 0.6 L/day above the basal osmolar excretory requirement of 0.5 and 0.75 L/day in younger and older individuals, respectively.

To add, I found this on Seeker.com; not sure how reliable it is, but it sounds plausible:

Drink the aqueous fluid found along the spine [cerebrospinal fluid] and in the eyes of large fish. Carefully cut the fish in half to get the fluid along the spine and suck the eye.

  • +1 I like your answer, and it seems reasonable. But this article about dolphins focus on salt as an issue not urea so I wonder if people have just been overlooking urea excretion in relatship to dehydration? – James Jenkins Sep 26 '18 at 13:42
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    If someone consumes about 60 grams of protein per day (the amount mentioned in the quoted part of my answer) he/she will excrete about 500 mL of urine as a result. We always consume some protein, so we are used to that and no one makes a problem from urea. But when you switch from 60 to 500 g of protein per day (in a fish-only-eating scenario), the amount of urea and hence urine greatly increases. – Jan Sep 26 '18 at 13:52
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    About salt: there is much lower amount of salt in fish than in seawater and it's comparable to other foods. With other words: neither the amount of salt and protein we usually consume result in dehydration, because we usually regularly consume some water as well. – Jan Sep 26 '18 at 13:55
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    So you would have to wring the water out of the fish? – Arsenal Sep 26 '18 at 16:45
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    @PaulBeverage, theoretically, drinking the "first batch" of urine could provide net hydration, but how much water can you get this way? A liter maybe. Because, as dehydration worsens, your urine will become more concentrated - it will contain more salt and urea - which can be excreted only with water, and at some point, drinking it will result in net water loss. No military or other survival manual I have read recommends drinking urine. outdoors.stackexchange.com/questions/264/is-drinking-urine-safe – Jan Oct 19 '18 at 12:58

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