Praise be to Allah.
What we have seen according to most of the authors of books of tafseer (Qur’aanic commentary) is that what is meant by the two seas here is the two well-known types of water that are found on earth:
(i)rivers of fresh water
(ii)seas of salty water
The evidence for this interpretation is the verse in which Allah says, describing the two seas (interpretation of the meaning): “one palatable and sweet, and the other salt and bitter” [al-Furqaan 25:53].
The evidence supports the view of the majority, unlike those who say that it refers to two seas, a sea in heaven and a sea on earth, or the seas of the Persians and Byzantines, or other strange opinions of which it cannot truly be said that one of them is palatable and sweet and the other is salt and bitter.
With regard to the barrier between the two seas that is mentioned in these verses, there are two scholarly views concerning it:
That what is meant by the barrier between the two seas (i.e., between rivers and seas) is the vast lands that separate the rivers from the seas, so that there is no mixing of their waters; rather each of them has its own course and destination that is separate from that of the other.
This is the literalist interpretation that we find with many of the commentators.
Al-Haafiz Ibn Katheer (may Allah have mercy on him) said: “and He has set a barrier and a complete partition between them” [al-Furqaan 25:53] means: between the sweet water and the salty water
“a barrier”, i.e., the dry land
“a complete partition” i.e., an obstacle that prevents one of them reaching the other.
End quote from Tafseer al-Qur’aan al-Azeem, 6/117
That between the two “seas”, the fresh and the salty, there is a barrier that is not visible to human eyes. Allah created it by His decree and it prevents mixing of the fresh water with the salty water even though the two waters meet at the mouths of rivers. This was narrated by al-Qurtubi from Ibn ‘Abbaas, and as-Suyooti attributed it to a report from ‘Abd ibn Humayyid from Qataadah. See ad-Durr al-Manthoor, 6/371
Imam al-Qurtubi (may Allah have mercy on him) said: “and has set a barrier between the two seas (of salt and sweet water)” [an-Naml 27:61] i.e., a barrier by His will, lest the salty mix with the fresh water. Ibn ‘Abbaas said: A barrier by His decree, so one does not change the other. End quote from Tafseer al-Qurtubi, 13/222
Al-‘Allaamah at-Taahir ibn ‘Aashoor (may Allah have mercy on him) said:
He created the barrier between the two seas in His great wisdom. It is a virtual barrier resulting from each type of water pushing against the other so that they do not get mixed, because of the relative difference in density due to the different components of salt water and fresh water.
So the barrier is made of both types, and it is not something different that separates them.
End quote from at-Tahreer wa’t-Tanweer, 20/13
There is no reason not to adopt both opinions in interpreting this verse, because each is correct and there is no contradiction between them. The barrier may indeed be dry land that separates rivers and seas, and it may be a virtual barrier (resulting from differences in density) which oceanographers speak of today. This is a different understanding, but there is no contradiction.
Dr Husayn al-Harbi says:
… The first type of different interpretation of the verse, which applies if all the different interpretations are possible for a particular verse and are supported by texts of the Qur’aan and Sunnah. This type of differences is possible and all the views are true and of equal worth because they are correct, and all these views are what the verse is referring to, and the Qur’aan testifies to each of these views.
End quote from Qawaa‘id at-Tarjeeh ‘inda al-Mufassireen, 1/42-45
And Allah knows best.