The Greek text reads:
Oute halikon glickie poisay hydoor.
In English that would be: "Neither does a spring of salt water manufacture sweet."
Literally the words are: NOT SALT [,] SWEET MANUFACTURE WATER.
That the text isn't speaking about whether water can be desalinated is clear from the first part which uses the same structure in stating: "a fig tree doesn't manufacture olives."
A person's mouth is compared to the fig tree and salty spring, and the question is what each produces or manufactures. The implication is that a person's heart is "salty" not "sweet" if the product is like what comes from a salty source. Besides Moses changed salt water to sweet according to the legendary narrative of the trek through the wilderness, so this is likely not a denial of what can be done with water.--Exodus 15.25.