Poisionous gas from Zoanthids + boiling water
The problem can be that you don't know you're sensitive until it's too late. Many people like myself seem to not have much sensitivity to coral toxins because of skin thickness or other factors. But my wife is the exact opposite.
When I use to take people on dives acting as Divemaster, I was always cautious when taking divers into coral gardens. First, I wanted to be sure they had accurate buoyancy control so they didn't go bouncing along destroying corals as they swam. Then, I asked them to not dive in gloves. Sounds counterproductive, but when they had gloves on, they didn't hesitate to hold onto rocks for stability and even touch corals. Without gloves, they touched very little, which was a good thing. But most everyone if diving near coral wore a protective skin or thin wetsuit, as there are a billion things in the sea that sting, eat or poison people.
We also had several tame Nassau groupers, some the size of a suitcase. People loved to pet them and they seemed to like it. It was far easier on their slime coats if you petted them with bare hands, so that was the rule.