Ultrasonic "steam" tends to aerosolize the total dissolved solids. Google "white dust" in regards to ultrasonic humidification. I suspect that this would not work for desalination.

Aside:

I have a home-built ultrasonic humidifier. If I run it with Boulder, CO tap water that is low in TDS, it only takes a day or so to have a PM2.5 >600 in my house. For this to work I had to install an RO filter in order to humidify with ultrasonic and not degrade air quality.

I don't think it matters whether you "boil" the water or not, you still need to put in enough energy to cause a state transition in the water you're evaporating. I see some papers on using ultrasonics to increase the efficiency of energy transfer from a heating element[1], but I don't think the second law of thermodynamics allows for a free lunch here.

[1] https://www.sciencedirect.com/science/article/abs/pii/S13594...

Edit: If you're thinking of something like an ultrasonic humidifier, I don't think these actually evaporate the water[2]. The mist these produce would still contain salt if you tried to use them for desalination.

[2] https://en.wikipedia.org/wiki/Humidifier#Ultrasonic_humidifi...

That will give you bone lung