Drinking saltwater does not hydrate the body. Instead, drinking excess amounts of saltwater, such as seawater, causes dehydration and may lead to death, states the National Oceanic and Atmospheric Administration. Dehydration results when the body tries to expel any excess salt through urination.
Saltwater or saline water contains different concentrations of dissolved salt or sodium. Water that is classified as very saline can contain from 10,000 to 35,000 parts per million of dissolved salt, while freshwater has less than 1,000 ppm, according to the United States Geological Survey.
The kidneys, which produces urine and filters chemicals like salt, cannot expel such high concentration of salt without sufficient freshwater to dilute it. Continuing to drink more saltwater causes more urination, leading to dehydration, notes Livescience.