Tafseer of the verse “He has let loose the two seas (the salt water and the sweet) meeting together. Between them is a barrier which none of them can transgress” [ar-Rahmaan 55:19-20]
Question

                In verses 19-22 in surah Ar-Rahman, Allah refers to two seas meeting side by side, and both of them having a barrier. What does this refer to?.
Praise be to Allah.
    Firstly: 

    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. 

    Secondly: 

    With regard to the barrier between the two seas that is 
    mentioned in these verses, there are two scholarly views concerning it: 

    -1-

    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 

    -2- 

    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.