What the heck do you even mean? What even is a water-caused death? I suppose humidity/heat related deaths might count, although those are tens or maybe hundreds

What the heck do you even mean? What even is a water-caused death? I suppose humidity/heat related deaths might count, although those are tens or maybe hundreds of thousands and hydrogen combustion would have a negligible contribution to the weather. Perhaps you mean overhydration? That's probably only a few dozen or so people unless you count deaths from MDMA-related overhydration, and even then it is still very few deaths overall. There are specific biological reasons that lead scientists to attribute certain deaths to air pollution. For instance, we know that PM2.5 can be absorbed through the lungs and we've seen that it forms masses in the veins and arteries that can cause heart attacks.