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.
The spa approach also achieves what’s probably the key ingredient to making this useful medically — consistent data over time. We know the “what’s this fuzzy bit!?!” hysteria with elective scans can be counterproductive, but if you truly had monthly scans going back 2 years, the fuzzy bit could in fact mean something.