Ash-Shaykh Bin Baaz rahimahullah was asked the following question,
A questioner inquires about the Tafsir (explanation) of the Saying of Allah (whose meanings are translated as):
‘He has let loose the two seas (the salt and fresh water) meeting together. Between them is a barrier. They do not transgress.’
Are those two seas in the worldly life or in the Hereafter?
These two seas are in worldly life. Different seas meet, but as Allah (Exalted be He) wants them apart, each remains separate and distinct; thus river water flows in its course and sea water in its course and between them there is a barrier so neither of them transgresses its limits. Rivers run as they are meant to be, fresh and sweet, whereas seas as they are meant to be, saline, and between them is a barrier, so neither of them transgresses and both of them are separated.” End quote. [Reference]
AbdulFattaah Bin Uthman