[Socrates]

Salt is the single most influential substance in the history of human civilization.

We all think of it as a table spice, but before freezing, salt was the only way to keep food fresh. Salt was more valuable than gold to the rise and fall of societies.

So I think the Salt Water versus Fresh Water comes from the fact that salt is heavily associated in our language and our past as preservative for non-fresh food.

Fun facts. Salary and Soldier are both Roman words derived from salt. Soldiers were paid in Salt regularly, and no army could be sustained or travel without salt. Now think about the phrasees that have salt in them like "he's really worth his salt".