Because freshwater only makes up about 3% of water on earth, a majority of water is salt water.
Obviously, this salt water is mostly in oceans and seas. So if freshwater makes up such a small amount of water on Earth, why don’t we start drinking salt water?
To put it simply, salt water would do a lot of harm to the human body. While the body needs salt, salt water has too high of a salt concentration for the body to process at once.
Additionally, there is not enough water to dilute the huge intake of salt. With constant drinking of salt water, you would eventually get dehydrated.
More information can be found on the National Ocean Service’s website.