Water Level Devices

The Water Level Devices are close-range devices that are typically used as evidence that the horizon is not at eye level. These devices are essentially a form of surveying, in which markers in the foreground are attempted to be aligned with something distant in the background.

Surface Tension

It has been found that the water in these devices are subject to surface tension and capillary action, and that the levels are not actually level.

From Encyclopedia Britannica we see:

Capillary Action

Further, it is apparent that capillary action needs to be taken into account.

Foreign Substances

It has been found that it is also possible for foreign substances, such as from alcohol or dye used in the experiment, to cause misalignment, since the weights of those fluids isn't consistent with the weight of the water.

From a problem on Chegg Study we find a depiction of unknown liquids floating on water:

Susceptible to Large Errors

It is seen that if the alignment of the water is not perfectly level, and that there is a slight imperfection, then a small misalignment in the foreground will create a large result in the background. The below illustration demonstrated the matter.

Essentially a very small angle misalignment with the line of sight of bodies in the foreground creates a very large angle numerous miles away.