I think it's a bit odd to compare this to an MRI. The physics are totally different and there are things it fundamentally won't image in the same way because it's basically just ultrasound.

The approach sounds like something which appears in a few research articles from the 2010s (ultrasound computed tomography), although submersion to make the ultrasound transmission more efficient seems novel.

It's possible the "spa" approach is used because it's hard to achieve the level of cleanliness required in a typical health facility using a shared bath.