"Water, water everywhere. Not a drop to drink." Many people are familiar with this simple piece of advice regarding seawater. But why can't people drink the water from the ocean? The short answer is the salt and saltwater from any source is generally undrinkable for people and animals. Although there are exceptions, the fact that saltwater will kill you should deter anyone from trying to drink it.
As almost any elementary school science student knows, two-thirds of the earth's surface is covered by water. Oceans, lakes and rivers dominate the planet's surface. However, 97 percent of that water is saltwater, which is undrinkable for people and most land animals. Even the 3 percent that is freshwater is not easily obtainable. About two-thirds of the freshwater in the world is frozen away in the polar ice caps and glaciers. Some of the rest is underground, and then the majority of what is left is in swamps. The good news, for water drinkers, is that the world's saltwater is mostly in the oceans, so what's on land is fresh.
The bad news is that if the only water around is saltwater, it's completely undrinkable. The simple reason for this is the salt. While it is true that many biological organisms--including people--need a certain amount of salt in their diet, the term "salt" itself is a generic descriptor for a whole category of minerals and compounds, not all of which are healthy. Table salt, for example, is not the same thing as what's found in the ocean. And the concentrations of salt found in seawater are much, much higher than what is needed for survival. A person's kidneys, which are normally responsible for removing impurities the body cannot process such as excess salt, are incapable of removing the amount of salt contained in saltwater.
All that extra salt in seawater means that, if drank, it causes the person (or animal) to suffer the effects of dehydration that much faster. Salt naturally dehydrates by absorbing water. This is why bars often put out salty peanuts or pretzels as free snacks. The kidneys respond to an increase in salt by taking more water from the body to flush out the extra salt. Salt makes patrons thirstier, so they order more drinks. However, the amount of salt in peanuts is much, much smaller than the amount found in saltwater. Aside from faster dehydration, which makes a person thirstier, drinking seawater also causes hallucinations, cramps and other health problems. Ultimately, death quickly ensues.
Of course, there are some animals that can and do drink saltwater. However, it should be noted that each of these animals has a special adaptation that allows them to drink it. The albatross, which is a large bird that spends most of its life at sea, has little choice but to drink saltwater. It also has a special gland near its eyes that secretes the extra salt. Dogs, on the other hand, are like people in that if they drink saltwater, it will overwhelm their kidneys and kill them. Without removing the extra salt, there is simply no safe way to drink saltwater.
There are a number of techniques for desalinating, or removing the salt, from seawater. Most of these involve some sort of evaporation process and are relatively simple if done with proper knowledge. Some life rafts on ocean-going boats have the equipment on board for desalination. As any survival expert will tell you, making sure the life raft is properly equipped should be a must before any excursion, no matter how short. There are also methods that can be used on land--on that tropical island beach, for example. Alternative sources of liquid, such as rainwater or even that tropical island cliche, the coconut, should be found whenever possible.