https://en.wikipedia.org/wiki/Cavitation

Cavitation is a phenomenon in which the static pressure of a liquid reduces to below the liquid's vapor pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure and as they rise,  these cavities, called "bubbles" or "voids", collapse and generate shock waves.  Non-inertial cavitation is the process in which a bubble in a fluid is forced to oscillate in size or shape due to some form of energy input, such as an acoustic field. Such cavitation is often employed in ultrasonic cleaning baths.

In an ultrasonic bath:  Microscopic gas bubbles that are generally present in a liquid will be forced to oscillate due to an applied acoustic field. If the acoustic intensity is sufficiently high, the bubbles will first grow in size and then rapidly collapse.

In industrial cleaning applications, cavitation has sufficient power to overcome the particle-to-substrate adhesion forces, loosening contaminants. The threshold pressure required to initiate cavitation is a strong function of the pulse width and the power input. This method works by generating acoustic cavitation in the cleaning fluid, picking up and carrying contaminant particles away in the hope that they do not reattach to the material being cleaned (which is a possibility when the object is immersed, for example in an ultrasonic cleaning bath).

CK Comment(s) :

The shock waves are termed plasma waves by industry experts. (I have used this term  since 2015). It is not bubbles hitting the item to be cleaned.  This is measured in PRESSURE using a cavitation meter and not power (current) draw.

The above answers why we need to degas water in an ultrasonic’ s  bath.