Two seas, a barrier between them

The ayah in tafsir

Maraja: He let them flow and meet. Barzakh: a partition. La yabghiyan: neither overflows its bound into the other so as to ruin it. Many mufassirun saw sweet river and salt sea, the estuary where they meet and yet the sweet remains usable. Some saw two salt seas. The ayah in al-Furqan names sweet and bitter plainly.

What later eyes saw

Where a great river meets the sea, density, temperature, and salinity can keep two bodies distinct for a long mixing zone — a halocline, a visible line in some gulfs. Divers later photographed what look like walls of water. The Quran named a meeting and a barrier that does not let one transgress the other.

Where we stop

Not every shoreline shows a hard wall. Estuaries mix. The verse is a sign of hudud — limits Allah set — not a claim that oceans never mingle a drop.