You have every right to be upset. Yes, there are many people/kids that like water balloon fights, but it wasn't a "fight" between just those that liked/wanted it. There was a person that was hurt and had no interest in the water balloons. He was just sitting on a bus, minding his own business.
I don't think that a letter to the editor was bad. It will allow the situation to be addressed. People will read it (and hopefully) see that they need to think before doing something. What if more people had this same idea and the letter changes their mind?
It could have been worse, the balloon could have damaged his eye (freak accidents do happen) and people need to think before they do something.
As for it being an assult, it was. Now, it was not intentional, but it was still an assult. Your child was hurt. Are you going to file a police report? That's your right, but I don't get from your postings that you want to do that.
I didn't read if you knew what parents did this, but if you do I would have a chat with them. There's not much that can be done now, but they need to know what they did was wrong.
Good Luck!