Sounds like you had quite the adventure. My guess is the more you do the route, the more you will be alert to these danger zones (like St Nicholas) and either avoid them or be prepared for the unexpected.

That said, the water balloon is kinda tough. It's not as bad as getting knocked off your bike by a huge water hydrant blast, but also not as refreshing! I remember riding back from Nyack last summer and up in that same neighborhood a bunch of hydrants were open. I think I was on Broadway, but any street would have been the same. Anyway, as I am coming down, I see this kid get a wicked glimmer in his eye - and I realize he wants to give me a "car wash" (this is where the kids divert the massive water flow from a hydrant with a bottomless cup or other implement to spray oncoming traffic.) Anyway, I was burning hot and thought - "why fight it" and kinda gave him a "go ahead" wave. (I think he would have done it even if I had told him he was a miscreant or some other word he wouldn't understand.) Turned out to be very refreshing (like a junior mint!) and so I went back for more, only the second time it was a direct hit and not only was I drenched (which was good) I was also laying on the pavement (which was not as good.) I hope the balloon cooled you down, but I realize all unwelcome (and unexpected) projectiles are cause for concern. That kid probably didn't have harm on his mind, but it's still potentially hazardous I suppose.