Bahrain in the Quran?
As described, one of these seas is salty, and the other is sweet. An interesting point made in this book, though, is that at least one of the references could involve the region called Bahrain, that is, the island nation of today and nearby coast of Saudi Arabia. Bahrain has lots of freshwater springs, some of which are actually on the floor of the Persian Gulf. These submarine springs have often been connected to the richness of pearl beds in that area, and Surah 55: 19-23 does mention pearls. To this, I'm tempted to also add 35:12, which refers to the extraction from the two seas of "ornaments which you wear." (The Arabic term translated as "both" is "kull," which can refer to a totality as well as counted units such as seas.)
A meeting place of the two seas is referred to in 18:60, in the context of a journey of Moses.