I used to work in ultrasound, and full body scans with the body underwater is definitely feasible and probably a good idea. Bit of a weird direction for them to take though??

Also there's absolutely no way that it will be as good as MRI. In general ultrasound imaging is shit. The main reasons it is used are because it is very cheap and completely harmless. The actual images you get are mostly just speckle. If MRI was cheap then nobody would use ultrasound. Full body ultrasound will definitely give better images because you have a wider aperture and can do fancier beamforming (probably "full matrix capture" and then beamforming in software; normally ultrasound probes do it in hardware). But it's still not going to be as good as MRI.

The exception to that is pregnancy - that is a super ideal case because you are imaging a nice clean interface in a fluid and there are no pesky bones in the way. Most of the body isn't like that at all.