How to recreate this scene?

[Abion47]

The biggest thing with these kinds of effects is the details. There are fishes and plants, sure, but there's also bubble particles, lighting effects, sunbeam effects, soundscape areas, fog areas, etc, etc. The list goes on and on. It's just as WALP said, the best thing to do is to open up the level file and just piece your way through it all. (You can destroy compounds as well to see what is inside them.)

One thing I can tell you with about 95% certainty is that there isn't any one button somewhere that says "make this area underwater".