[chris]

Or better still, diabetes insipidus...

[JnA]

so if you are stuck at sea the better thing to do would be to try to catch a seagull or fish and drink the blood?

[CliffordK]

The best thing to do is to try to find a way to capture, recover, or distill fresh water including capturing the morning dew if there is some.

Actually...  I would really consider getting some kind of a portable desalinator put in my liferaft    But, I guess one may not always have control of the situation leading to being stranded.
http://www.247water.org/seapack.html
http://www.247water.org/seapack_crew.html
http://www.campingsurvival.com/kahaemdewama.html

I would be a bit worried about the "disposables"   What if one needs more water.  If they only do a quart or two, it might be best just to stock the liferaft with a few gallons of clean water instead.

I was thinking a bit about the new sailboats with bilge tanks.  There certainly would be a reason to fill the bilge tanks with Fresh Water rather than Salt Water, especially if it could be compartmentalized and consumed one tank at a time.

Here is a "solar still" on Wikipedia.
http://en.wikipedia.org/wiki/Solar_still


The idea looks pretty simple, and should be able to be adapted to a boat assuming one had some kind of pastic or tarp available.  The main difference would be to either distill bilge water, or distilling it from moistened clothing.


And, yes, FOOD is also good.  But, I think I'd consider a bird, fish, or a piece of seaweed as just food, and not trying to suck their blood.  Heck, if I thought I could eat bird feathers, I'd probably chew on them too.

Keep in mind that saltwater fish may have saltier blood than humans.

[CliffordK]

I thought I'd look up some of the numbers...  it is a little hard to find everything, but, looking at Sodium and Chloride ions.  Note, the urine also is the primary balance for Nitrogen (Urea), and other water soluble toxins.  But regulation of Sodium (salt) is important when considering it with respect to sea water.

Sodium in Blood 136–145 mEq/L
Hypernatremia is supposed to have a 70% mortality rate with levels greater than 160 mEq/l with the highest levels in literature being 180 or 190 mEq/l
Sodium in Urine max of about 270–290 mEq/l
Chloride (similar to Sodium) in Sweat < 40 mEq/L  (while you think it is salty, that is only because of evaporation, it is actually hypotonic.

Sodium in Sea Water is 468 mEq/l
Sodium in Great Salt Lake or Dead sea would be about 10x that much.

Drinking sea water will replace fluid in your body...  but it is at an extreme cost as your body won't be able to recover its salt balance.
The sea water enema will likely cause some salt to be absorbed, and create an osmotic gradient likely causing a tendency to loose water.  However, it seemed like a good idea, somewhat like the very poor man's peritoneal dialysis.
Your bladder doesn't concentrate fluids, so injecting the salt water up your willie wouldn't help either.  And the best way to get fluids to the kidneys is by ingesting them.
Your own urine may have a lower concentration of sodium than the sea water...  But it will have a higher nitrogen concentration, and it is still at the point where your kidneys can't make it any more concentrated.

As mentioned, there are diseases that affect the urine.
The Classic uncontrolled Type 1 Diabetic will have urine resembling Gatorade including sugars, and because of an osmotic gradient, less salt.
Diabetes insipidus also causes unconcentrated urine.

The problem is that the individuals with (primary or secondary) urinary disorders causing dilute urine would be in very very very bad shape.

[thedoc]

We put this to Miles Parks, Gastroenterologist, and Ari Ercole, Intensivist, both from Addenbrookes Hospital...

Miles -   Using the rectum as a means of administering fluid replacement in dehydrated individuals is an interesting idea.  The colon or large bowel functions primarily to absorb fluid, normally doing so of course as material enters to the cecum from the small intestine, the material which has not been digested, fibrous products and so on, together with a substantial amount of fluid and solute enters into the colon, and the fluid is then sucked out as the material goes around the 4 feet or so of the colon to form up the stool.  And so, you can see the colon is well designed for absorbing liquid but it really needs to do so in the context also of transport of solutes, that’s to say of sodium and chloride ions, and so on.  It’s the absorption of these, of the solute itself which then creates the osmotic gradient which sucks fluid across the lining of the bowel, and into the bloodstream.  So, bearing these things in mind, I do think that water enemas on their own or water on its own is perhaps unlikely to be absorbed in a clinically significant quantities, and of course, it’s just likely to come out of the rectum again whereas, I think administering saline or something of that type would potentially lead to quite a significant absorption of fluid.

Diana -   Water on its own probably wouldn’t do very much, but a saline solution could give one a better chance of a drink.  But what if you're stuck for saline drips and all you've got is a much saltier seawater?

Ari -   Unfortunately, giving yourself a seawater enema for hydration if you've had the misfortune to be stranded at sea is a complete thermodynamic nonstarter.  The problem is that the salt in seawater is much more concentrated than the concentration of all the various solutes found in body tissues.  Since, to a first approximation at least, the gut can be thought of as a sort of semipermeable membrane, this will lead to water molecules tending to move from the body through the gut wall, and into the seawater to reduce the concentration difference.  This process where water moves along its own concentration gradient across a semipermeable membrane is called osmosis, and it’s very important in biology.  In this case it will actually result in you becoming increasingly dehydrated.  The situation is reversed with freshwater which would be successfully absorbed.  Having said that, neither procedure would be very safe especially if the water was dirty.  So, giving yourself a seawater enema if you're trapped at sea is likely to make you lose water.  Of course, giving yourself a seawater enema if you're trapped at a cocktail party is likely to make you lose friends as well.