Air is free because there is more than enough of it for everyone, everywhere.
Water isn't like that, and there is no way to make it.
Air isn't free as such, either, because of pollution. The moment you pollute air you are degrading its quality, up to the point that certain equipment has to actively filter the air before using it. Also keep in mind that "pollute" for example in denser settlements can also mean smells, e.g. someone is constantly cooking stuff that strongly smells and you live "downwind" of that, would you consider air as "free" or should there maybe some limit to what can be done with it?
Why would you compare it to art, instead of steel or wheat?
I was clarifying the kind of non-commodity that decommodification of resources refers to