Must have been a crappy windshield. Obviously it wasn't the force of the balloon hitting the windshield.. it was the difference in temperature of the water and the windshield itself - though most winshields should be built to withstand that.. Otherwise, hitting a bird could kill people.
OK, how much does a baseball weigh? I don't honestly know, but I'd have a hard time believing it was more than a water balloon of equal size. A MLB pitcher can throw a baseball at 100 mph. Would you believe that could break a windshield? How about a baseball thrown at 50 mph?
That much mass moving at any decent speed is a lot of energy.
Viper GTS