It's probably easier to put the sonic generator unit into an enclosed space like crew quarters.
I don't think replicating water would be an option, because it would probably use more energy than a sonic. Plus the water would still have to come from somewhere - replication doesn't create things out of nothing.
Although regarding sonic showers, I still keep wondering how they would actually work. I can see how high frequency waves would physically get the dirt off you, but do they actually get you CLEAN? Do they take care of the sweat and the stink? I just don't see how they could do that. I'm sure it actually feels much better to take a water shower than a sonic one.