I don't think it's just water, it's water and cleaning solution of some type.
For the underneath of your car it's probably safe and a great way to get the underneath of your car rinsed off at least to some level.
For the outside paint though, just keep in mind that if nothing mechanical is touching the paint to loosen built-up dirt and the film we call road grime, then something chemical must be taking it's place together with high pressure spray.
This is just a guess but a guess that I've been posting for years and I've never seen anyone add to it or take away from it, if nothing mechanical is touching the paint, that is a brush or anything, then the chemicals used to remove dirt and road grime are probably strong enough to reduce any amount of protection you have on the finish also.
Maybe not, so this is just a guess...
In a harsh winter though, if your car is really dirty, then a touch-less would be a better option then the Swirl Master Brush Beast type car car washes at it would be easier to replace a coating of wax or paint sealant over time then it would be to remove all the swirls and scratches out of the paint, (and remove a little paint in the process), like Jeff demonstrated in this before and after detailing project.