I have to recommend a cool book on this ... Why Geese Don't Get Obese. It delves into the biological systems of different animals and explains how they do some amazing things. For instance, there was a chapter on how fresh water fish and salt water fish differ in how they drink water.
Thanks, I was just wishing that this article would have gone into that.
Presumably whales and other marine mammals can drink ocean water -- how else would that work? -- so it's not an inevitable design constraint that our internal salinity is less than that of the ocean. Presumably our ancestors evolved to drink low-salinity water (and eat separate sources of salts, and automatically mix the correct salts into the water in the correct proportions, and maintain those proportions at all costs -- lots of overhead!) because fresh water is what's plentiful in our habitat. In other words, it's because lakes, rivers, and springs yield fresh water that we've evolved all these mechanisms for tolerating fresh water.
Another nice poetic question is: why are we made of so much water? The answer is that life evolved in the ocean. We're built out of crucial parts that were developed in the ocean and that can't operate outside of a saltwater environment. So life colonized the land by evolving a way to carry its own ocean with it. ;) I'm a giant EVA unit for ocean-dwelling cells!
For example: house cats are believed to be descended from a desert cat. They have adaptations that make sense for a desert environment, such as dry poop, concentrated urine and very efficient kidneys. A house cat could survive on brackish to salty water for quite a while. You and I could not.
