OK, here's the place ya need to be in TMO (this is actually FotRSU, but the same scene)
You will want to use that middle gauge.
Here's a closer view of that little middle gauge
Click about where you want to set your depth
... and here we are at our set depth...
Back "safe" on the surface
If you do not have that gauge like that, maybe a different background color, then something isn't correct in the mod.