It's a question as old as humanity: Even though we are surrounded by the stuff, why can't humans drink seawater? The human need for drinkable water is second only to breathable air in the biological hierarchy, so it would make sense to evolve to take advantage of this nearly inexhaustable resource.
But nearly all land mammals evolved to drink the more scarce of the two kinds of water on Earth and, unfortunately for us, lost the capacity to drink seawater in the process, if we ever even had it to begin with.
What's more, this is going to be a much more important issue in the decades ahead, since our need for fresh water rather than the salty kind has locked us into a constant struggle to find fresh water to drink exactly where it is least plentiful — on land. That struggle is only going to become more difficult as climate change melts glaciers and dries up rivers and lakes, making an already scarce resource even more so.
Why Can't Humans Drink Salt Water?
The problem with drinking seawater is that for land mammals like us, drinking salt water actually causes dehydration. There are a few land mammals that are known to occassionally drink seawater, like otters and sea lions, but even marine mammals like whales and dolphins don't rely on water from the ocean to stay hydrated internally.
The issue, according to the US Geological Survey, is that in order to maintain a healthy balance of salt in our blood, any excess salt needs to be filtered out by our kidneys and converted into urine.
Since our body can't produce urine that is saltier than our blood, and typical ocean water has three times the salt of our blood, our kidneys would actually need to produce a greater volume of urine than the volume of water we took in by drinking seawater.
To get that water, the kidneys would pull freshwater from other available sources in our bodies, like our cells, and this can lead to dehydration and death in pretty short order.
So why can some animals drink seawater?
There are some animals, like gulls, sea lions, and albatrosses, that can drink some level of seawater without issue. As the American Museum of Natural History notes, in the case of the albatross, it actually has a special gland that filters out the salt from the water it gulps up, and secretes it from its beak.
Meanwhile, according to Scientific American, examinations of the urine of seals and sea lions show a much saltier urine than your typical human, as much as two and a half times as salty as seawater, or about seven to eight times saltier than their blood. This may have something to do with the way their kidneys are structured, so they are able to produce a much more concentrated salt content in their urine than is typical of most land mammals.
When it comes to marine mammals like whales, dolphins, or manatees, there is some debate on exactly how much saltwater they drink, though they almost certainly drink some amount of seaware. Examinations of manatee kidneys indicate longer internal tubes, called Henle loops, that are essential to reclaiming water. Since whales and other ocean mammals are more difficult to study than land-dwelling mammals, researchers do not know yet for certain.
In all cases though, all mammals get some water from the foods they eat, humans included, some just get more of their water from food than others. In the case of California sea lions, a study showed that they can get all the water they need just from eating fish.
Other sea-dwelling mammals can produce sufficient fresh water internally from the metabolic breakdown of food (water is a by-product of carbohydrate and fat metabolism).
The typical human, meanwhile, only gets about 20% of their required water from food, according to WebMD, and we lack the processes necessary to survive by getting 100% of our water from food. In fact, water is important for our ability to digest and process the food we eat.
Why preserving fresh water sources is so vital
All of this is why the issue of water scarcity brought on by climate change is such a crisis. Without reliable access to fresh drinking water, humans and nearly every other land mammal will have little recourse to get the water we need, and species will face extinction as they are forced to abandon their traditional habitats.
There is no easy answer to this problem, either. Evolution works too slowly for us to produce kidneys that are more efficient at extracting salt from our blood, so we're not going to develop the ability to drink salt water any time soon. Meanwhile, water desalination plants have been around for ages, but desalination is incredibly energy-intensive, so this isn't really a practical solution to provide all of the water needs for life on Earth.
There are plenty of solutions that might work on a smaller scale, for example, Saudi Arabia is building large desalination plants powered by solar energy, but the scale of the crisis is simply so much greater than any of our solutions have been able to truly address. In the end, the only way out of this crisis may be to retreat from it as best we can. By cutting emissions and forestalling the worst effects of climate change, we won't completely avoid the problem of water scarcity, but it will be more manageable. Otherwise, we may find ourselves dying of thirst and staring out at the ocean, choking on the bitter irony.