Koran 35:12

And not alike are the two seas [i.e., bodies of water].

One is fresh and sweet, palatable for drinking, and one is salty and bitter.

And from each you eat tender meat and extract ornaments which you wear, and you see the ships plowing through [them] that you might seek of His bounty; and perhaps you will be grateful.
Koran 35:12

This is part of the Two Seas in the Koran. For more, see: Koran – Two Seas?

There is no fresh water sea. Muhammad didn’t understand the water cycle.