The Most Gracious - 55:20 There is a barrier between them; they do not transgress and interfere with each other.
Related:
The Criterion/25:53 He has released the two seas, this one fresh and sweet, this one salty and bitter. He has placed between them a strong barrier preventing them from mixing.

The Ants/27:61 Who made the earth a dwelling place, and caused rivers to flow between them, and placed firm mountains on it, and placed a barrier between the two waters? Another god besides Allah? Verily, most of them do not know.

Originator/35:12 The two seas are not equal to each other. This one is fresh and sweet and soft to drink, and that one is salty and bitter. From each of them you eat fresh meat and extract jewels to wear. You will see ships splitting it, seeking His favor. Perhaps now you will give thanks.

The Most Gracious/55:19 He has released the two seas; they meet each other.
The Most Gracious/55:20 There is a barrier between them; they do not transgress and interfere with each other.
The Most Gracious/55:22 Pearls and coral come out of both of them.