Heard an interesting talk yesterday about coral, and remote sensing of water temperatures and light levels as a means of tracking how they're doing. The effort was prompted by the major coral bleaching events in the last decade. I'm a physical, rather than biological, oceanographer, so my prior knowledge of corals is well-covered by a) coral bleaching events are bad for coral and b) coral are pretty. A good entry point on the web for the coral monitoring efforts is NOAA Coral Reef Watch, where you can find out much more.
One of the things that is important for coral is water temperatures. If water gets too hot, it's bad for the coral. That's old news at this point. The addition from this presentation was that coral also care about light levels. If it's too bright, that's also bad for the coral. They can adapt to some degree, over time, to high light levels.
As I suggest in the title, the situation is not simply those two things. High temperatures aren't good. But if the lighting isn't too strong, it's survivable. The presentation included observation of a time that had prolonged high temperatures, but the lighting wasn't very strong and the coral survived ok. A situation that didn't have as high temperatures (though still high) but did have excessive lighting resulted in much more bleaching.
It's also the case for coral, as for people, that it is sustained extreme conditions which matter. So, again, the concern is for heat waves, not hot individual days.