In fact, you do not even have to strike someone to commit an assault under common law. Simply raising your hand in a manner that would cause a reasonable person to fear an offensive touching is enough.
I love it when internet blowhards attempt to apply the law, especially when they have no concept as to what the law actually means, or how it is actually applied.
In reality, most people think that their own fantasies are law, and that the world owes it to them to apply their "law" and only their "law." That's not the system that our Framers set up. Like it or not.
In this case, preparing the throw the water balloon is the assault and hitting the LEO is the battery.
Justin Beiber, what a wimpy little twerp. Any other place he would be receiving Texas wedgies in the high school locker room.