If someone started yelling and swearing at you on the bus can you pull a water gum and spray water in their face? or maybe do it with a bottle of water.
Would you get arrested for that?
It would be a crime and a tort. Battery. If you touch a person with something you cause to contact them rather than with your fist, for example, you have still committed the crime and tort.
I remember reading one extreme case where I believe a tortfeasor was found liable (i.e. it was a civil case and he was sued and had to pay damages) for battery for blowing smoke in someone's face.
There may also be some creating-a-commotion or having-liquid-on-the-bus-without-a-lid or other random laws and rules implicated in your example.