Now, tossing any kind of object from aircraft has always been a no no under FAR's, though I would imagine that most of us, have, at one time or another, tossed "objects" out of either an open window, or over the side of an open cockpit. It is hoped we all did this over a safe area, and chose our impact points with some care..... The actions of these idjits with water balloons were, IMHO, criminal, and since they were allegedly student pilots, that is as far as they should get.... Akthough a balloon full of water may not seem like much, one only has to figure out the mass and estimated terminal velocity to see that one winds up with a lethal missile. Such items are OK in contests along with flour bags, etc, etc at carefully supervised events, and carefully chosen impact points, clear of any danger, but these dumb yahoos deserve the full impact of the law.
I once witnessed a similar idiot throw a pair of wood chocks out of the window of a C180 for the alleged reason "there was a junk car and I wanted to see if I could hit it"... Well, the guy obviously had no concept of speed, trajectory, or exterior ballistics, because the damn chocks came close to impacting the writer, and his car, as well as a C210 on which I was doing a preflight check and which was a good 300 yards away from the intended "target". The aircraft was NOT mine, it was for a charter,I had passengers standing by the aircraft, and the chocks missed it by less than 5 yards. I lost the charter, as the pax suddenly decided they would rather not fly, then had and had some severely terrified people to calm down. To protect all concerned, I will not mention the airport, nor the state, After said idiot landed, I had a "extended discussion" with this pilot, in which I "explained the error of his ways" to him in no uncertain terms, and left the chocks hanging around his neck. No further report was needed, as said pilot never flew from the airport again. (Fear is a wonderful motivational tool) This type of behavior has no place in modern aviation, except in airshows and some fun contests, where safety regs are in place to make sure that nothing gets damaged.
It is far too bad that we have this kind of abject stupidity more in more in GA today, especially with the advent of the "Sport PIlot" rating, which IMHO, should never have been instituted, not in today's high density airspace situation, and most especially, in the ever increasing restrictions and controlled airspace popping up every day since 9/11....... Just my .02, even if, without a current medical, and current experience, I can't sit in the pointy end any more, but one doesn't forget one's experience and how one feels about idiots creating a hazard for themselves and others. Fly safe!! Greg