Water covers about 70 percent of the surface of the planet, and almost all of that water is seawater, more or less synonymous with ocean water. (A sea is an inland "piece" of saline water, which is unusual since most ponds and lakes are considered freshwater.) As a result, humans have come up with a lot more things to do with seawater than float around on it or stare appreciatively at the rolling waves lapping at a favorite beach.

You can't just grab a bucket of seawater and find an immediate practical use for it, as you certainly can't drink it as it is extremely hypertonic to your own body fluids (more on that scary-looking word in a moment). But with the right tools and ingenuity, you can put some of this massive reserve of less-than-pure H₂O to excellent use.

While you may naturally think of the atmosphere as Earth's main storage depot for gases such as carbon dioxide and oxygen, the ocean below it is in fact a far more massive reservoir, producing half of the planet's oxygen and storing about 50 times as much carbon dioxide as the atmosphere itself. Since plants use CO₂ for fuel while generating oxygen as a waste product and animals do the reverse, this is critical for the ordinary maintenance of Earth's biological balance.

The seas provide the United States alone with close to $300 billion in revenue every year, and over three-fourths of American trade involves the oceans in some way or another.

• Freshwater has less than 1,000 ppm (parts per million) of NaCl in water. Slightly saline water ranges from 1,000 ppm to 3,000 ppm. Moderately saline water ranges from 3,000 ppm to 10,000 ppm. Finally, highly saline water ranges from 10,000 ppm to 35,000 ppm

• About 13 percent (a little over one-eighth) of the water in the U.S. is saline, including one of the world's largest inland seas, the Great Salt Lake in the state of Utah. While its salinity makes it unusable in some ways, this doesn't limit its use in, for example, the mining industry, in which over half of the water uses is salt water.
  
  
• "Deep sea water" (DSW) is a term that refers to water obtained from a depth of 200 m (about 656 feet) or more below the ocean surface.
  
  
• In 2018, the use of water in the U.S. fell to its lowest level in 48 years, continuing a decline that began in 2010.
  
  
• About 3 million Americans are employed as a result of industries centering on marine activities.

The use of salty water is made easier by desalination, which can be done through either distillation or osmosis. Making seawater potable, or drinkable, is plainly a useful goal, but the process is expensive and therefore usually only performed on a large, industrial scale.

From a health standpoint when it comes to salt water, facts are that some evidence exists that salt water benefits include the prevention of atherogenesis (the formation of "plaques" in the arteries that carry blood to the tissues of the body). It also may be helpful in treating the symptoms of skin conditions such as eczema, and may also prevent some of the damage to the liver that may result from a high-fat diet.

Deep sea water may be effective in the prevention of certain cancers, including breast cancer. The mechanism for this is not well understood as of 2020, but research efforts in the area of cancer prevention using DSW continue. It is also believed that DSW can mitigate some of the symptoms of diabetes mellitus.