The Qur’an speaks of two oceans; one salty and one sweet. While it is assumed that this means the salty oceans and the fresh rivers, it is just that: an assumption. God uses words precisely in the Qur’an. The Arabic word baḥr is usually translated as ocean, sea or a large body of water. The Qur’an overwhelmingly uses the singular and dual forms of baḥr. There is a reason for this consistency.
Is He who appointed the ground a settlement and appointed in its midst rivers and appointed for it anchors and appointed between the two oceans a partition.1 Is there a god with God? No, but most of them do not know (27:61)
And it is He who loosed the two seas, this palatable, sweet and this salty, bitter. And He appointed between them a barrier and a prohibiting prohibition (25:53)
We are left with two questions after reading the above verses: What are the two oceans and what is the partition between them? We can establish from the above that one ocean is salty and the other sweet. Now, science informs us that there are numerous rivers and oceans in the earth. But if we actually look at this claim carefully, we find that there is really one salty ocean encircling the ground; all the oceans – Pacific, Atlantic etc. – are interconnected so they actually are just one body of water i.e. one salty ocean. As for the rivers, it is clear that what the Qur’an means by a sweet ocean cannot mean rivers for three reasons: Firstly, there are many unconnected rivers so they cannot comprise one interconnected body of water. Secondly, there is no ‘prohibiting prohibition’ between the rivers and the ocean; most of them drain into it. Thirdly, the Qur’an uses a different word for rivers: ‘anhar.
And not equal are the two oceans; this palatable, sweet and this salty, bitter. And from each you eat tender meat and extract adornments to wear. And you see the ships sailing in it that you might seek of His bounty and that you may be grateful (35:12)
The picture starts to become clear. Besides the salty ocean, there exists a sweet ocean: a massive underground reservoir accessible via lakes, springs, aquifers and wells. From the lakes which are a part of this reservoir, we catch fish. This is the sweet ocean being referred to in the Qur’an. The sweet ocean is constantly being replenished by means of rain.
Have we not appointed the ground a restrainer. Living and dead? (77:25-26)
Mirza, Said. Cosmology of the Qur’an: The Seven Skies and the Fixed Ground (p. 49-50). Men of God. Kindle Edition.