What I'm wondering is why humans can't drink the water from the most ubuquitous reservoir on earth.
So even though much of the planet is covered with sea water very few mammals have developed the ability to use sea water for hydration. I just find that surprising.
The thing is that we need a fairly careful balance between salt and fluid in our bodies, too much salt and our nervous system gets upset.
True, but we ingest all sorts of substances in amounts that we don't need. The rest is usually excreted unless having a store of it is useful (like with fat).
Apparently there was not enough evolutionary pressure and that is what I find very surprising.
https://en.wikipedia.org/wiki/Cetacea
There is this bit:
"The kidneys are long and flattened. The salt concentration in cetacean blood is lower than that in seawater, requiring kidneys to excrete salt. This allows the animals to drink seawater."
So for those animals that returned to the sea from the land that pressure was there and they seem to have adapted in that sense.
Humans - and most other mammals - being land dwellers we always had access to rivers and rainfall both of which are fresh.