For the edge between the surface and water, an invisible cube is rendered to texture for postprocess. The box is binded to the camera by xz coordinates only.
The size of the cube is only a couple of meters, i.e. smaller than the size of the water itself, and this is enough for the boundary.
The scene with the cube is black, the outer side of the cube is painted black, the inner side is white.
The black color in the post effect displays the surface, the white color displays the underwater world. When diving into the cube, part of the front side of the cube and the back side of the cube are naturally displayed, thereby displaying the water boundary.
