To your question, yes you can vary the amplitude of the square (or sine) wave to the Piezo transmitter.
You could use an LM317 controlled to a specified voltage. You could use the PWM output of the Arduino (with RC filtering) to control the LM317.
What I’m not sure of is the specification 720mW/cm2. Is it at the input of the Piezo (likely not) or the energy coupled to the transmitting medium? If so how is it evaluated?
If you are trying to pass energy through some (non air) medium be aware that any changes in the Speed of Sound (SOS) in the mediums with reflect energy back to the transducer.
For instance: If you are going through human tissue. You have to go through fat and muscle. I’m going to assume the SOS in fat is not = to the SOS in muscle. So when the piezo induced wave travels through the Fat - muscle interface, some of the energy will be reflected back to the transmitting piezo. The amount reflected back is proportional to the change in SOS between the two mediums.