I dove on a fresh water upwelling in the Gulf of Mexico. When the dive master told us what it was, I took my regulator out and swallowed some of it. It was clean and pure.
That reminds me of a Clive Cussler book when Dirk Pitt traveled through a underground river through the desert and into the gulf of Mexico or Baja.
Fun book.
I read an account many years ago, about a ship that went down off the coast of California. Everyone made it to the lifeboat and were saved. But they were way off from nearest land and no potable water. So they prayed and asked for a miracle. Almost immediately the water beneath their boat became fresh. It was like “an oasis”, the captain later described it as being.