A Barrier That is Not Crossed

S 25:53 And he is the one who mixed the two seas. This one freshwater, sweet water, and this one salt, salty. And he put between them an isthmus and a forbidden enclosure.

S 55:19-20 He mixed the two seas, meeting. Between them an isthmus, they do not violate.

The word that I have translated 'isthmus' is barzakh. It is defined:

بَرْزخ : - حاجِز

- Something that protects from something else - A substance that does not conduct heat - A narrow piece of land connecting two larger bodies of land - bar; partition; screen; divider; isthmus

المختصر

بَرزَخ : - فاصِلٌ زَمَنِيّ، وَقْت

- time; interval; break; pause https://www.almaany.com/en/dict/ar-en/%D8%A8%D9%8E%D8%B1%D9%92%D8%B2%D9%8E%D8%AE%D9%8C/

Ibn Kathir for S 25:53

(And it is He Who has let free the two seas, this is palatable and sweet, and that is salty and bitter;) means, He has created the two kinds of water, sweet and salty. The sweet water is like that in rivers, springs and wells, which is fresh, sweet, palatable water. This was the view of Ibn Jurayj and of Ibn Jarir, and this is the meaning without a doubt, for nowhere in creation is there a sea which is fresh and sweet. Allah has told us about reality so that His servants may realize His blessings to them and give thanks to Him. The sweet water is that which flows amidst people. Allah has portioned it out among His creatures according to their needs; rivers and springs in every land, according to what they need for themselves and their lands.وَهَـذَا مِلْحٌ أُجَاجٌ(and that is salty and bitter;) meaning that it is salty, bitter and not easy to swallow. This is like the seas that are known in the east and the west, the Atlantic Ocean and the Straits that lead to it, the Red Sea, the Arabian Sea, the Persian Gulf, the China Sea, the Indian Ocean, the Mediterranean Sea, the Black Sea and so on, all the seas that are stable and do not flow, but they swell and surge in the winter and when the winds are strong, and they have tides that ebb and flow. At the beginning of each month the tides ebb and flood, and when the month starts to wane they retreat until they go back to where they started. When the crescent of the following month appears, the tide begins to ebb again until the fourteenth of the month, then it decreases. Allah, may He be glorified, the One Whose power is absolute, has set these laws in motion, so all of these seas are stationary, and He has made their water salty lest the air turn putrid because of them and the whole earth turn rotten as a result, and lest the earth spoil because of the animals dying on it. Because its water is salty, its air is healthy and its dead are good (to eat), hence when the Messenger of Allah was asked whether sea water can be used for Wudu', he said:«هُوَ الطَّهُورُ مَاؤُهُ، الْحِلُّ مَيْتَتُه»(Its water is pure and its dead are lawful.) https://quranx.com/Tafsir/Kathir/25.51

For S 55:19-20 Allah created Different Types of WaterAllah said,مَرَجَ الْبَحْرَيْنِ يَلْتَقِيَانِ(He has Maraja the two seas), or let them loose, according to Ibn `Abbas. Allah's statement,يَلْتَقِيَانِ(meeting together.) Ibn Zayd said, "He prevents them from meeting by the dividing barrier He placed between them to separate them.'' The two seas are the fresh and salty waters, the former coming from running rivers. We discussed this topic in Surat Al-Furqan when explaining Allah's statement;وَهُوَ الَّذِى مَرَجَ الْبَحْرَيْنِ هَـذَا عَذْبٌ فُرَاتٌ وَهَـذَا مِلْحٌ أُجَاجٌ وَجَعَلَ بَيْنَهُمَا بَرْزَخاً وَحِجْراً مَّحْجُوراً(And it is He Who has let free the two seas: one palatable and sweet, and the other salty and bitter; and He has set a barrier and a complete partition between them.)(25:53) Allah said,بَيْنَهُمَا بَرْزَخٌ لاَّ يَبْغِيَانِ(Between them is a barrier which none of them can transgress.) meaning, He has placed a barrier of land between these two types of waters, so that they do not transgress upon each other, which would spoil the characteristics they were created with. Allah said,يَخْرُجُ مِنْهُمَا الُّلؤْلُؤُ وَالمَرْجَانُ(Out of them both come out pearls and Al-Marjan. ) pearls are well-known. As for Marjan they say it means small pearls. Mujahid, Qatadah, Abu Ruzayn, Ad-Dahhak said it, and it has also been reported from `Ali. It was also said that it means large, precious pearls, this was mentioned by Ibn Jarir from some of the Salaf. Ibn Abi Hatim recorded from Ibn `Abbas who said, "When it rains, the oysters in the sea open their mouths. What falls in them, the drops, turns into pearls.'' Its chain of narrators is Sahih. https://quranx.com/Tafsir/Kathir/55.14

It is a land barrier. Verse 22 of S 55 says: Pearls and marjan come out of them.

A sea cannot be of freshwater. There are apparently freshwater pearls. Marjan is ambiguous. Definition:

1. small pearls

2. corals https://en.m.wiktionary.org/wiki/%D9%85%D8%B1%D8%AC%D8%A7%D9%86

Coral cannot survive in freshwater.