What's the use of FWI for a moving target - which someone floating in a spa most definitely will be?

This is a fever dream, not a practical diagnostic technology.

I can imagine it being useful if you happened to own an imaging product and wanted to train a model on human body shapes and sizes.

But dunking people in a golden spa - why golden? why not pink or teal? - and waving a ring of US over them, at distances which make high resolution impractical, to tell them their medical fortunes is... eccentric.

I'm being scathing because there's no solid science behind this, and if there had been solid science MJ would have published it in a paper.

This is pure marketing. And if you're going to do that, you should at least include some dolphin petting.