[fauigerzigerk]

That was hundereds of thousands of years ago and we (well, many of us) have adapted to drinking milk (as adults) during that time.

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.

[thenewwazoo]

Processing milk is a digestive change, and one that mammals are all able to do at some point. It’s not a very far leap. Processing water with the sea’s salinity would require changes to how our cells are constructed, or at least would require massive changes to our kidneys. There’s simply no (or not enough) selection advantage for it to have happened.