Perspective only.
Centered vertical and horizontal sighting lines, both fore and aft:

Leveling of cube is essential:

If cube is plumb and square to level, obtaining a view or picture with fore and aft horizontal sighting lines aligned means the horizontal line is on a plane of "eye level"
Lines of perspective drawn from cube wires should converge somewhere on that line. If they don't, it means pitch isn't level. If lines converge below horizontal, then the cube is pitched forward and the camera is too high with focal plane is sloped downward. If they converge above horizontal, the cube is pitched back and the camera is too low with focal plane sloped upward.
With fore and aft vertical sighting lines also aligned, the lines of perspective should converge on the vertical line (less important for horizon measurement.)
Lines of perspective drawn from the cube wires should converge somewhere on the vertical line if camera centered laterally (and cube level). If lines converge left or right of the vertical line, then the camera is off center and the cub yawed left or right. This isn't critical for measuring horizontal plane, but to get all lines to converge on the center crossing of the vertical and horizontal sight lines, it is.

Debated with Tom previously about lack of importance of camera centering or orientation (pitch/yaw/roll). For this, it is significant if the pivot point isn't the center of the camera body; which it isn't. The pivot point for this tripod is below, and so when the camera is tilted forward, backward or side to side, it will in slight ways alter the height of the camera. (Yaw, or panning, without changing pitch or roll has no effect on height).
This isn't as simple as the water gauge. To make this work, the cube must be level in both fore/aft and left/right axes. The viewpoint must then be level with the horizontal sight lines. That establishes the eye level plane benchmark against which a horizon line is compared. Post editing of video image with lines of perspective will verify cage orientation if lines converge as expected.
Since leveling is best accomplished with a plumb bob, minimal wind is optimal. But even without a plumb bob, a bubble level checking multiple axes is sufficient. Just as the water "levels itself," the sight lines and the perspective lines can only line up correctly in one orientation. If something doesn't appear right, something is not level or aligned. It just takes more time to calibrate.
I'm going to treat the water level as a separate tool and remove it from the cube.