There are 2 different physical sensations for tasting salt. One for low salt that tastes good and one for high salt that tastes bad. This is believed to help regulate salt intake.
Slightly salty water shouldn't taste bad. Soda and Gatorade are both relatively salty. If you are dehydrated, you might even prefer water with a dash of salt. In fact, Dasani et Al, carefully control the salt balance to make the flavor right.
But yeah. Water is tasteless so a pinch of salt goes a long way when there is no fat or protein (or sugar) to cut it.
Dasani and Smart Water have high salt to mineral content, they're my favorite brand of bottled water. Pure water tastes bad and is actuality dangerous to drink regularly
Soup is mostly salty water and tastes great. If you are trying to drink water with electrolytes and the taste bothers you, a squeeze of lemon will fix it, or one of my kids uses a splash of cranberry juice instead.
Evolutionary, I'd expect it's because drinking saltwater is generally counter productive.
If you are on a raft without water, you will die faster id you drink the salty sea water than if you don't.
Drinking lots of slightly salty water, will put stress on your kidneys, as fresh water is the remedy to compensate if you got too much salt.
That's the evolutionary perspective, but how the mechanics work for the body to detect it IDK. You can have watery sauce or soup that is salty but taste great, despite being mostly water. So I think there is a bit of what's expected involved. We generally don't expect water to be salt.
The perception of saltiness in food and water can vary due to how it interacts with other flavors. Salty foods often have a balance of salt that enhances the overall taste experience. Conversely, salty water can taste unpleasant because the salt can overpower your water’s flavor by itself, making it less enjoyable. Unless you mean sea water though, that has a lot more stuff in it.