Question:
I put bottled water into the battery. I wasn't expecting anything except watch the electrolyte level top up. But I am sure I heard a click and there was certainly a sudden change (darkened) in the color of the battery plates. I took a measurement of of voltage and it was totally zero. Even an old "dead" battery shows minimal signs of a voltage but mine might as well be a piece of lead. Zilch. Attempts to recharge it were unsuccessful.

What can there be in the mineral water that can destroy a battery so effectively and instantly?

Answer: Boiling the water does nothing to help it. It's the minerals that can make the battery soupy. They'll get in the way of the normal lead-acid chemistry and they can also drain the battery by constant electrolysis. You'll see such a battery bubbling even when it hasn't been used or charged in days. The constant charging needed to overcome that drain wears out the plates.

What you saw was your battery shorting out. You stirred up a bunch of lead crud and that either broke a weak plate or it caused a short.

Next time use distilled water. The battery will last longer.

You can look up battery chemistry in Google. It's fairly simple but I forget the details of the oxygen motion.