I might be simplifying things here, but my understanding is that most sea creatures (notably fish) can “drink” salted water. Most (probably all) mammals, birds and even insects can’t. Water is pretty much essential to life as we know it on Earth, salt is pretty much essential to life too. Salted water is abundant. What made “us” lose the ability to drink it? Even more when you consider that fresh water is often a cause of diseases due to pathogenic bacterial.
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Sea creatures have specialized kidneys to allow them to drink high concentrations if salt water. Most land animals don’t have this ability, and can only drink small amounts before it’s potentially dangerous.
It’s not that we lost the ability to drink salt water. Most fish have a gland the excretes excess salt. Not being fully submerged in salt water all the time means that we didn’t need that gland (if we ever had it) so evolution did not act upon it. The first organisms on land that lost the gland possibly had an advantage meaning more offspring were produced without it and eventually animals completely lost the gland.
Most sea animals don’t drink the water either. They hydrate thru their food or topically (? Not sure on the wording). The few that do have special adaptations.
So we didn’t lose anything. We need many minerals to survive, some of which can come from the ocean but it was never needed for us to drink the water.
Fresh water was available on land in enough quantity to support life. As you go inland, fresh water is more abundant compared to salt water.
Also, drinking sea water consumes more energy than fresh water as your body has to do the removal of salt. That energy could be spent better somewhere else.
Saltwater animals have specialized kidneys and other organs (like salt glands or active sodium pumps in their gills) to keep the internal sodium levels in check. Once ancestors of land or freshwater animals left the salty sea those adaptations were no longer needed and were lost during evolution as not only they were not needed, but could also be detrimental to an organism survival on land, where salt is scarce. It also drains energy to keep them running, so across the ages those adaptations were evolved out of the gene pool like any other redundant organ.
I am not a biologist, but I’m rather sure no mammals can drink salt water for their water intake.
This is due to a process called Osmosis, where concentrations of two volumes of water with different concentrations of electrolytes, separated by a membrane, will equalize.
Because the concentration in our cells is lower than the salt water concentration, our cells will lose water until they cannot survive any longer.
Iirc salt water fish have a higher salt concentration inside their cells.
This is the reason why saltwater fish die in freshwater bodies. Their cells will “explode” because much water has to be added in the cells to equalize.
I’d guess that the desalinisation process (in the kidneys according to u/NathanTheZoologist below) is energy intensive enough that it was simply more taxing on the body or whatever progenitor was drinking than fresh water.