Just to be a bit pedantic, I thing the gas created it H2SO4 or hydrogen sulfide (or is that H2S its been a while since I cracked a chem text). Mucho volatile.

More importantly...

This illustrates why it is vital that when jump starting ANY vehicle, the last connection of the cables should be a negative connection and should be at a good ground location far from the battery itself. The final connection will always create some sort of spark and lead-acid batteries create this gas. Imagine hanging over the battery one jumper in hand when that lets rip -- talk about your chemical peels. (Not to mention permanent blindness, life long scars, and, if you happen to be inhaling at the time, upper airway swelling and sudden death... Just food for thought.

On adding "tap water" to a battery. I'm not a purist prude by any stretch, I use zerex in the rad and chevron motor oil but: tap water -- no matter how good it tastes will shorten the life of the battery. Impurities introduced to the fluid will encourage crystals to precipitate out of it and sit on the bottom of the cell. Once they accumulate and touch the lead plates the battery is kaput. A gallon of distilled is what, 3 bucks at the drug store? If you use five percent of that topping up a wet cell I'd be surprised.