It looks like the actual location point of your Underwater Post Process is probably considered to be the center of the cube that represents the swimmable area. So if you leave the z position at 0, it will be halfway underground and halfway above. It could be that the location point is at the top of the swimmable cube. If that's the case, you may want to raise it up even farther to 500, if your size height is set to 5.
The cube I'm talking about is the volume box for the Underwater Post Process. I tried it out in mine and the location point on the volume box is located on the top of the box. (look at the where the gizmo is located)
When I make the z value for scale 5, then I need the position to be set at 500 if I want the volume located right on top of the ground. (If set for 8 then it needs to be 800, set at 6 it needs to be 600 etc.)