Yeah, that compression wave bothered me too. That indicates displacement and given the apparent volume of liquid it should be difficult for Voyager to push against that much mass (which made me ask my original question about crush depth). Also, usually if there's no external atmospheric pressure on a liquid in a vacuum or near vacuum it will start to boil into a vapor but we weren't shown anything like that. It would be funny if we were talking about a huge scale difference and all of 8472's universe existed on just a microscope slide in some other reality.Of course pressuse density is another matter entirely, but it seemed to be negligible other than sending a compression wave.
In the end I think I have to put in the same category as when one of the Q transformed Voyager into a christmas tree ornament - we'll just have to smile and nod.