Good post - my wife and I moved in December, so this is our first summer in the house. We planted a handful of hydrangeas and some are doing better than others. Its been a rough year for my plants: first being transplanted, then bouts of heavy rain and pretty consistent high humidity, followed by the influx of heat you mentioned. As the person attending to the plants, it can be confusing because the signs of drought and drowning are similar, and I could be wrong, but I often feel I see both: my Bobo dropped its lower leaves, which I thought was due to too much wetness post-transplant (my little lime did the same), but it nearly melted in the heat the other day. I try not to water right when I get home from work because hydrangeas are "dramatic": they look like a person about to faint in an old black-and-white movie by the mid-afternoon, but they often recover when their roots manage to catch up and suck up enough water to replace the water that evaporated during the afternoon. I think that often leads to overwatering: when people get worried about a little afternoon dehydration. I try to water in the morning if the plant looks thirsty and a hot day is coming - they generally have not needed water beyond that.

Yeah, inconsistent weather is awful.

We've not had a drop of rain since the remnants of Harvey!

We went more than 95 days without any measurable precipitation (more than 1/10 of an inch) - from mid June to just around the middle of September. While we normally have very dry summers, this one was unique in its duration. And we have only had about 1/2" total since the rains returned.........not a great deal. I watered inground hydrangeas once a week (quite deeply) and containerized ones every other day.