Seawater makes up the oceans and seas, covering more than 70 percent of Earth's surface. Seawater is a complex mixture of 96.5 percent water, 2.5 percent salts, and smaller amounts of other substances, including dissolved inorganic and organic materials, particulates, and a few atmospheric gases. On an average, seawater in the world's oceans has a salinity of about 3.5% (35 g/litre or 35 parts per thousand - ppt). This means that every kilogram (roughly one liter by volume) of seawater has approximately 35 grams of dissolved salts, predominantly sodium and chloride ions. The average density of sea water at the surface is 1.025 kg/litre. The seawater is denser than both fresh water and pure water (density 1.0 kg/litre at 4 °C) because the dissolved salts increase the mass by a larger proportion than the volume. The freezing point of seawater decreases as salt concentration increases. At typical salinity, it freezes at about −2 °C. The coldest seawater still in the liquid state ever recorded was in a stream under an Antarctic glacier, in 2010; the measured temperature was −2.6 °C .
Why the sea water is salty? The water in the sea was thought to come from the Earth's volcanoes, starting 4 billion years ago, released by degassing from molten rock (degassing, also known as degasification, is the removal of dissolved gases from liquids, especially water or aqueous solutions). Scientific theories behind the origins of sea salt started with Sir Edmond Halley in 1715, who proposed that salt and other minerals were carried into the sea by rivers after rainfall washed it out of the ground. Upon reaching the ocean, these salts concentrated as more salt arrived over time. Halley noted that most lakes that don't have ocean outlets (such as the Dead Sea and the Caspian Sea), have high salt content. Halley termed this process as "continental weathering". Halley's theory was partly correct. In addition, sodium leached out of the ocean floor when the ocean formed. The presence of salt's other dominant ion, chloride, results from outgassing (which is the release of a gas that was dissolved, trapped, frozen, or absorbed in the bottom of the sea) of chloride (as hydrochloric acid) with other gases from Earth's interior via volcanos and hydrothermal vents. The sodium and chloride ions subsequently became the most abundant constituents of sea salt. The salinity levels are less than 0.05 % in fresh water, 0.05 -3.0 % in brackishwater (semi-saline water), 3.0 -5.0 % in saline water and more than 5 % up to 28 % maximum in brine solution.
Ocean salinity has been stable for billions of years, most likely as a consequence of a chemical/tectonic system which removes as much salt as is deposited; for instance, sodium and chloride sinks include evaporite deposits, pore-water burial, and reactions with seafloor basalts.
Although the vast majority of seawater has a salinity of between 31 g/kg and 38 g/kg, that is 3.1–3.8%, seawater is not uniformly saline throughout the world. Where mixing occurs with freshwater runoff from river mouths, near melting glaciers or vast amounts of precipitation (e.g. Monsoon), seawater can be substantially less saline. The most saline open sea is the Red Sea, where high rates of evaporation, low precipitation and low river run-off, and confined circulation result in unusually salty water. The salinity in isolated bodies of water can be considerably greater still; about ten times higher in the case of the Dead Sea. Is it okay to drink seawater? Accidentally consuming small quantities of clean seawater is not harmful, especially if the seawater is taken along with a larger quantity of fresh water. However, drinking seawater to maintain hydration is counterproductive; more water must be excreted to eliminate the salt (via urine) than the amount of water obtained from the seawater itself. In normal circumstances, it would be considered ill-advised to consume large amounts of unfiltered seawater. A summary of 163 life raft voyages estimated the risk of death at 39% for those who drank seawater, compared to 3% for those who did not. The effect of seawater intake on rats confirmed the negative effects of drinking seawater when dehydrated The temptation to drink seawater was greatest for sailors who had expended their supply of fresh water, and were unable to capture enough rainwater for drinking. This frustration was described famously by a line from Samuel Taylor Coleridge's The Rime of the Ancient Mariner:
"Water, water, everywhere,
And all the boards did shrink;
Water, water, everywhere,
Nor any drop to drink."
Most oceangoing vessels desalinate potable water from seawater using processes such as vacuum distillation or multi-stage flash distillation in an evaporator, or, more recently, reverse osmosis. These energy-intensive processes were not usually available during the Age of Sail. Larger sailing warships with large crews, such as Nelson's HMS Victory (1765 – 1922), were fitted with distilling apparatus in their galleys. Animals such as fish, whales, sea turtles, and seabirds, such as penguins and albatrosses have adapted to living in a high saline habitat. For example, sea turtles and saltwater crocodiles remove excess salt from their bodies through their tear ducts.
Desalinization, the removal of salt from seawater offers great promise to supply citizens in arid regions or have irregular precipitation and are near the ocean in the future to come. However, due to its energy consumption, desalinating sea water is generally more costly than fresh water from surface water or groundwater, water recycling and water conservation. Desalination processes are usually driven by either thermal (in the case of distillation) or mechanical (in the case of reverse osmosis) as the primary energy types. Based on the requirement and economic status of the country desalinization is currently in vogue. There are approximately 16,000 operational desalination plants, located across 177 countries, which generate an estimated 95 million m3/day of freshwater. Currently, desalination accounts for about one percent of the world's drinking water. Desalination is particularly prevalent in countries located in the Middle East and North Africa region, such as Saudi Arabia, the UAE, and Kuwait. According to International Desalination Association, desalination produces 21 billion gallons of water per day, providing a crucial water source in arid places. It claims that 300 million people get water from desalination and their numbers are quickly rising. In the coastal region of India, a few seawater desalination plants are in operation using reverse osmosis. As per the Desalination Association of India, there are more than 1000 membrane-based desalination plants of various capacities ranging from 20 m3/day to 10,000 m3/day. With the continuous decrease in the availability of freshwater the option to make use of seawater would be high in the years to come.