Yeah... that's exactly right.
The problem with the "Trek" incarnation of these things is that they're inconsistent with the REAL science it's based upon. Trek "sonic showers," supposedly, use no water. This is illogical and irrational, but not too surprising... the folks writing about it most often had no idea of the science behind it.
The idea of a sonic shower, as TGT said, isn't that it's WATERLESS... but that it's "water-efficient." Basically, the "sonic" part simply provides a very efficient SCRUBBING action of the soapy water... getting the job done without requiring ongoing, constant running water. It still does require water, and soap. THAT is what the Treknology writers seem to have gotten all confused about.
As for headaches and seizures and so forth... it has to do with intensity and frequency. You'd need to closely control the sonic excitation frequencies used. I mean... ultrasonic scanners (which, as the name implies, use high-frequency sonics) are by far the safest way to look at developing fetuses, for instance, and it's implied that many of Dr. McCoy's sensing devices are sonic-based technologies. It's unlikely that a sonic device would be inherently hazardous... though it WOULD be possible to sabotage the system and cause injuries to the crew (possibly even injuries that wouldn't show symptoms immediately, only over a significant length of time... which, if you think about it, would make for some interesting storytelling possibilities!)