Amazingly and surprisingly, such a place exists — where two different seas meet, but can never combine because each has its own identity (temperature, density..etc.)
In the northern part of Denmark there is a place called Skagen. It’s a very small town with population less than 10 K. There is a sea beach called Grenen, which is a tree branch-like sand formation, and at the tip of it, you can stand on one foot and you’ll be at the barrier of land where North Sea meets the Baltic Sea. Cool huh?
The photo above shows L to R: Baltic Sea, Denmark, North Sea